I also still see this problem on Ubuntu Server 9.04 running 2.6.28-11. Is this the patched Intrepid kernel, or do I need to enable backports or do some other thing to resolve this issue?

I have a RAID 5 running with 3 disks across two controllers:
VIA Technologies, Inc. VIA VT6420 SATA RAID Controller (rev 80)
Silicon Image, Inc. SiI 3512 [SATALink/SATARaid] Serial ATA Controller (rev 01)

/var/log/messages:
[  590.274538] ata1: hard resetting link
[  590.594161] ata1: SATA link up 1.5 Gbps (SStatus 113 SControl 310)
[  590.819220] ata1.00: configured for UDMA/100
[  590.819266] ata1: EH complete
[  590.848089] sd 0:0:0:0: [sda] 1250263728 512-byte hardware sectors: (640 GB/596 GiB)
[  590.854831] sd 0:0:0:0: [sda] Write Protect is off
[  590.861069] sd 0:0:0:0: [sda] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA
[  607.360924] ata1: hard resetting link
[  607.679134] ata1: SATA link up 1.5 Gbps (SStatus 113 SControl 310)
[  607.695647] ata1.00: configured for UDMA/100
[  607.695693] ata1: EH complete
[  607.700588] sd 0:0:0:0: [sda] 1250263728 512-byte hardware sectors: (640 GB/596 GiB)
[  607.700662] sd 0:0:0:0: [sda] Write Protect is off
[  607.700764] sd 0:0:0:0: [sda] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA
[  637.277044] ata2: hard resetting link
[  637.596839] ata2: SATA link up 1.5 Gbps (SStatus 113 SControl 310)
[  637.829733] ata2.00: configured for UDMA/100
[  637.829780] ata2: EH complete
[  637.844617] sd 1:0:0:0: [sdb] 1250263728 512-byte hardware sectors: (640 GB/596 GiB)
[  637.853532] sd 1:0:0:0: [sdb] Write Protect is off
[  637.858736] ata2: hard resetting link
[  638.176875] ata2: SATA link up 1.5 Gbps (SStatus 113 SControl 310)
[  638.194087] ata2.00: configured for UDMA/100
[  638.194139] ata2: EH complete
[  638.198664] sd 1:0:0:0: [sdb] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA
[  638.201640] sd 1:0:0:0: [sdb] 1250263728 512-byte hardware sectors: (640 GB/596 GiB)
[  638.201737] sd 1:0:0:0: [sdb] Write Protect is off
[  638.201967] sd 1:0:0:0: [sdb] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA

*However*, disabling the write cache does not fix this for me:

/var/log/messages:
[ 1387.362851] sd 0:0:0:0: [sda] 1250263728 512-byte hardware sectors: (640 GB/596 GiB)
[ 1387.363165] sd 0:0:0:0: [sda] Write Protect is off
[ 1387.363330] sd 0:0:0:0: [sda] Write cache: disabled, read cache: enabled, doesn't support DPO or FUA
[ 1387.363638] sd 0:0:0:0: [sda] 1250263728 512-byte hardware sectors: (640 GB/596 GiB)
[ 1387.363748] sd 0:0:0:0: [sda] Write Protect is off
[ 1387.363897] sd 0:0:0:0: [sda] Write cache: disabled, read cache: enabled, doesn't support DPO or FUA
[ 1416.321442] ata2: hard resetting link
[ 1416.641237] ata2: SATA link up 1.5 Gbps (SStatus 113 SControl 310)
[ 1416.874078] ata2.00: configured for UDMA/100
[ 1416.874124] ata2: EH complete
[ 1416.914095] ata2.00: configured for UDMA/100
[ 1416.914149] ata2: EH complete
[ 1416.928420] sd 1:0:0:0: [sdb] 1250263728 512-byte hardware sectors: (640 GB/596 GiB)
[ 1416.938974] sd 1:0:0:0: [sdb] Write Protect is off
[ 1416.939283] sd 1:0:0:0: [sdb] Write cache: disabled, read cache: enabled, doesn't support DPO or FUA
[ 1416.939525] sd 1:0:0:0: [sdb] 1250263728 512-byte hardware sectors: (640 GB/596 GiB)
[ 1416.939592] sd 1:0:0:0: [sdb] Write Protect is off
[ 1416.939778] sd 1:0:0:0: [sdb] Write cache: disabled, read cache: enabled, doesn't support DPO or FUA

This is happening even when just reading files, so perhaps I need to disable the read cache or is this a bad idea? I couldn't find the option in hdparm for disabling the read cache.

This is a brand new array with only test data on it so far, so I am happy to try any tests (provided they will not cause hardware damage :-)) and provide any more required details if necessary. If this is a separate problem to the original post, let me know and I can open another bug report.