I'm trying to install Lucid, Kubuntu 10.04 beta 2, and I think I'm having the same problem.
Ubiquity hangs when scanning disks. The progress bar stops at 47%, and never go further.

At the same time, the process ksoftirqd/0 uses 100% of the cpu.

Here are the logs from ubiquity:
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/var/log/syslog
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[...]
Apr 13 11:21:45 ubuntu activate-dmraid: No Serial ATA RAID disks detected
Apr 13 11:21:47 ubuntu kernel: [ 1612.911257] sr 8:0:0:0: timing out command, waited 28s
Apr 13 11:21:59 ubuntu kernel: [ 1625.200403] end_request: I/O error, dev fd0, sector 0
Apr 13 11:22:11 ubuntu kernel: [ 1637.280025] end_request: I/O error, dev fd0, sector 0
Apr 13 11:22:56 ubuntu kernel: [ 1682.186393] INFO: task udisks-daemon:3110 blocked for more than 120 seconds.
Apr 13 11:22:56 ubuntu kernel: [ 1682.186398] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
Apr 13 11:22:56 ubuntu kernel: [ 1682.186401] udisks-daemon D 0000000000000000     0  3110   3109 0x00000000
Apr 13 11:22:56 ubuntu kernel: [ 1682.186407]  ffff8800c0b49c18 0000000000000082 0000000000015b80 0000000000015b80
Apr 13 11:22:56 ubuntu kernel: [ 1682.186413]  ffff880115fa03c0 ffff8800c0b49fd8 0000000000015b80 ffff880115fa0000
Apr 13 11:22:56 ubuntu kernel: [ 1682.186418]  0000000000015b80 ffff8800c0b49fd8 0000000000015b80 ffff880115fa03c0
Apr 13 11:22:56 ubuntu kernel: [ 1682.186423] Call Trace:
Apr 13 11:22:56 ubuntu kernel: [ 1682.186433]  [<ffffffff8153f3a7>] __mutex_lock_slowpath+0xe7/0x170
Apr 13 11:22:56 ubuntu kernel: [ 1682.186438]  [<ffffffff8153f29b>] mutex_lock+0x2b/0x50
Apr 13 11:22:56 ubuntu kernel: [ 1682.186443]  [<ffffffff81172ef0>] __blkdev_get+0x60/0x3d0
Apr 13 11:22:56 ubuntu kernel: [ 1682.186447]  [<ffffffff81173280>] ? blkdev_open+0x0/0xc0
Apr 13 11:22:56 ubuntu kernel: [ 1682.186451]  [<ffffffff81173270>] blkdev_get+0x10/0x20
Apr 13 11:22:56 ubuntu kernel: [ 1682.186454]  [<ffffffff811732f1>] blkdev_open+0x71/0xc0
Apr 13 11:22:56 ubuntu kernel: [ 1682.186459]  [<ffffffff81140823>] __dentry_open+0x113/0x370
Apr 13 11:22:56 ubuntu kernel: [ 1682.186464]  [<ffffffff8125177f>] ? security_inode_permission+0x1f/0x30
Apr 13 11:22:56 ubuntu kernel: [ 1682.186468]  [<ffffffff8114cbaf>] ? inode_permission+0xaf/0xd0
Apr 13 11:22:56 ubuntu kernel: [ 1682.186472]  [<ffffffff81140b97>] nameidata_to_filp+0x57/0x70
Apr 13 11:22:56 ubuntu kernel: [ 1682.186477]  [<ffffffff81150dea>] do_filp_open+0x2da/0xba0
Apr 13 11:22:56 ubuntu kernel: [ 1682.186482]  [<ffffffff8105b1b5>] ? wake_up_process+0x15/0x20
Apr 13 11:22:56 ubuntu kernel: [ 1682.186488]  [<ffffffff810c90a3>] ? rcu_start_gp+0x53/0x1a0
Apr 13 11:22:56 ubuntu kernel: [ 1682.186492]  [<ffffffff8115e390>] ? mntput_no_expire+0x30/0x110
Apr 13 11:22:56 ubuntu kernel: [ 1682.186496]  [<ffffffff8115c89a>] ? alloc_fd+0x10a/0x150
Apr 13 11:22:56 ubuntu kernel: [ 1682.186500]  [<ffffffff81140599>] do_sys_open+0x69/0x170
Apr 13 11:22:56 ubuntu kernel: [ 1682.186504]  [<ffffffff811406e0>] sys_open+0x20/0x30
Apr 13 11:22:56 ubuntu kernel: [ 1682.186509]  [<ffffffff810131b2>] system_call_fastpath+0x16/0x1b
Apr 13 11:23:47 ubuntu kernel: [ 1732.921257] sr 8:0:0:0: timing out command, waited 120s
Apr 13 11:24:56 ubuntu kernel: [ 1802.190033] INFO: task udisks-daemon:3110 blocked for more than 120 seconds.
Apr 13 11:24:56 ubuntu kernel: [ 1802.190037] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
Apr 13 11:24:56 ubuntu kernel: [ 1802.190040] udisks-daemon D 0000000000000000     0  3110   3109 0x00000000
Apr 13 11:24:56 ubuntu kernel: [ 1802.190046]  ffff8800c0b49c18 0000000000000082 0000000000015b80 0000000000015b80
Apr 13 11:24:56 ubuntu kernel: [ 1802.190052]  ffff880115fa03c0 ffff8800c0b49fd8 0000000000015b80 ffff880115fa0000
Apr 13 11:24:56 ubuntu kernel: [ 1802.190057]  0000000000015b80 ffff8800c0b49fd8 0000000000015b80 ffff880115fa03c0
Apr 13 11:24:56 ubuntu kernel: [ 1802.190063] Call Trace:
Apr 13 11:24:56 ubuntu kernel: [ 1802.190072]  [<ffffffff8153f3a7>] __mutex_lock_slowpath+0xe7/0x170
Apr 13 11:24:56 ubuntu kernel: [ 1802.190077]  [<ffffffff8153f29b>] mutex_lock+0x2b/0x50
Apr 13 11:24:56 ubuntu kernel: [ 1802.190082]  [<ffffffff81172ef0>] __blkdev_get+0x60/0x3d0
[...]
Apr 13 11:24:56 ubuntu kernel: [ 1802.190140]  [<ffffffff81140599>] do_sys_open+0x69/0x170
Apr 13 11:24:56 ubuntu kernel: [ 1802.190143]  [<ffffffff811406e0>] sys_open+0x20/0x30
Apr 13 11:24:56 ubuntu kernel: [ 1802.190149]  [<ffffffff810131b2>] system_call_fastpath+0x16/0x1b
Apr 13 11:24:56 ubuntu kernel: [ 1802.190157] INFO: task parted_devices:11703 blocked for more than 120 seconds.
Apr 13 11:24:56 ubuntu kernel: [ 1802.190160] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
Apr 13 11:24:56 ubuntu kernel: [ 1802.190162] parted_device D 0000000000000000     0 11703  11702 0x00000000
Apr 13 11:24:56 ubuntu kernel: [ 1802.190167]  ffff88009c267c18 0000000000000086 0000000000015b80 0000000000015b80
Apr 13 11:24:56 ubuntu kernel: [ 1802.190173]  ffff8800a55d5f80 ffff88009c267fd8 0000000000015b80 ffff8800a55d5bc0
Apr 13 11:24:56 ubuntu kernel: [ 1802.190177]  0000000000015b80 ffff88009c267fd8 0000000000015b80 ffff8800a55d5f80
Apr 13 11:24:56 ubuntu kernel: [ 1802.190182] Call Trace:
Apr 13 11:24:56 ubuntu kernel: [ 1802.190187]  [<ffffffff8153f3a7>] __mutex_lock_slowpath+0xe7/0x170
Apr 13 11:24:56 ubuntu kernel: [ 1802.190191]  [<ffffffff8153f29b>] mutex_lock+0x2b/0x50
Apr 13 11:24:56 ubuntu kernel: [ 1802.190195]  [<ffffffff81172ef0>] __blkdev_get+0x60/0x3d0
[...]
Apr 13 11:24:56 ubuntu kernel: [ 1802.190230]  [<ffffffff8115c89a>] ? alloc_fd+0x10a/0x150
Apr 13 11:24:56 ubuntu kernel: [ 1802.190234]  [<ffffffff81140599>] do_sys_open+0x69/0x170
Apr 13 11:24:56 ubuntu kernel: [ 1802.190237]  [<ffffffff811406e0>] sys_open+0x20/0x30
Apr 13 11:24:56 ubuntu kernel: [ 1802.190242]  [<ffffffff810131b2>] system_call_fastpath+0x16/0x1b
Apr 13 11:25:47 ubuntu kernel: [ 1852.931258] sr 8:0:0:0: timing out command, waited 120s
[...]
Apr 13 11:28:45 ubuntu kernel: [ 2030.961258] sr 8:0:0:0: timing out command, waited 28s
Apr 13 11:30:45 ubuntu kernel: [ 2150.970006] sr 8:0:0:0: timing out command, waited 120s
Apr 13 11:31:56 ubuntu avahi-daemon[1445]: Invalid query packet.
Apr 13 11:32:15 ubuntu kernel: [ 2240.980009] sr 8:0:0:0: timing out command, waited 90s
[...]
Apr 13 11:41:45 ubuntu kernel: [ 2811.050007] sr 8:0:0:0: timing out command, waited 120s
Apr 13 11:43:15 ubuntu kernel: [ 2901.060005] sr 8:0:0:0: timing out command, waited 90s


----------------------------------------------------
/var/log/installer/debug
----------------------------------------------------
[...]
Bus::open: Can not get ibus-daemon's address. 
IBusInputContext::createInputContext: no connection to ibus-daemon 
QSocketNotifier: Invalid socket 19 and type 'Read', disabling...
QSocketNotifier: Invalid socket 19 and type 'Exception', disabling...
QSocketNotifier: Invalid socket 19 and type 'Write', disabling...
QSocketNotifier: Invalid socket 19 and type 'Read', disabling...
QSocketNotifier: Invalid socket 19 and type 'Exception', disabling...
QSocketNotifier: Invalid socket 19 and type 'Write', disabling...
QSocketNotifier: Invalid socket 19 and type 'Read', disabling...
QSocketNotifier: Invalid socket 19 and type 'Exception', disabling...
QSocketNotifier: Invalid socket 19 and type 'Write', disabling...

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/var/log/partman
----------------------------------------------------
[...]
/lib/partman/init.d/30parted: IN: OPEN =dev=sdb /dev/sdb
parted_server: Read command: OPEN
parted_server: command_open()
parted_server: Request to open =dev=sdb
parted_server: Opening outfifo
parted_server: OUT: OK
parted_server: OUT: OK
parted_server: Note =dev=sdb as unchanged
parted_server: Closing infifo and outfifo
parted_server: main_loop: iteration 3
parted_server: Opening infifo