At 2am, after working away happily for 1.5 days (I copied the work out regularly), some time after a 45Mbyte Software Update, it
began to go wrong. A reboot prompted the following repair, and I left it:

[    6.314129] EXT4-fs (sda1): orphan cleanup on readonly fs
[    6.314137] EXT4-fs (sda1): ext4_orphan_cleanup: deleting unreferenced inode 309788
[    6.315820] EXT4-fs (sda1): ext4_orphan_cleanup: deleting unreferenced inode 766985
[    6.317334] EXT4-fs (sda1): ext4_orphan_cleanup: deleting unreferenced inode 769166
[    6.317369] EXT4-fs (sda1): ext4_orphan_cleanup: deleting unreferenced inode 769170
[    6.317391] EXT4-fs (sda1): ext4_orphan_cleanup: deleting unreferenced inode 768775
[    6.320236] EXT4-fs (sda1): ext4_orphan_cleanup: deleting unreferenced inode 814440
[    6.320273] EXT4-fs (sda1): ext4_orphan_cleanup: deleting unreferenced inode 768541
[    6.322899] EXT4-fs (sda1): ext4_orphan_cleanup: deleting unreferenced inode 311047
[    6.322945] EXT4-fs (sda1): ext4_orphan_cleanup: deleting unreferenced inode 977877
[    6.322973] EXT4-fs (sda1): ext4_orphan_cleanup: deleting unreferenced inode 310939

I left it until this morning, and at 9am  after a restore of my work by copying it in, and a little use of Chrome:

[  665.062016] rm[14080]: segfault at 4 ip b77ab60d sp bfbd20a0 error 4 in ld-2.15.so[b77a9000+20000]
[  665.160702] mv[14084]: segfault at 4 ip b7790481 sp bfa935b0 error 4 in ld-2.15.so[b7785000+20000]
[  665.264448] touch[14088]: segfault at 4 ip b77c9481 sp bf82cac0 error 4 in ld-2.15.so[b77be000+20000]
[ 1348.241462] dell_wmi: Received unknown WMI event (0x11)
[ 7654.917714] readlink[14150]: segfault at 4 ip b776a481 sp bfeedef0 error 4 in ld-2.15.so[b775f000+20000]
[ 7655.051813] dirname[14156]: segfault at 4 ip b76fc481 sp bff23930 error 4 in ld-2.15.so[b76f1000+20000]
[ 7655.182041] mkdir[14163]: segfault at 4 ip b77b2481 sp bfe16680 error 4 in ld-2.15.so[b77a7000+20000]
[14836.922957] readlink[14226]: segfault at 4 ip b7784481 sp bfd0e0c0 error 4 in ld-2.15.so[b7779000+20000]
[14837.011050] dirname[14230]: segfault at 4 ip b77c5481 sp bfc2a6a0 error 4 in ld-2.15.so[b77ba000+20000]
[14837.107422] mkdir[14235]: segfault at 4 ip b775e481 sp bf91e510 error 4 in ld-2.15.so[b7753000+20000]
[15019.548968] EXT4-fs error (device sda1): __ext4_ext_check_block:472: inode #310825: comm rs:main Q:Reg: bad header/extent: invalid magic - magic 8b1f, entries 8, max 20596(0), depth 20597(0)
[15019.548974] Aborting journal on device sda1-8.
[15019.549058] EXT4-fs (sda1): Remounting filesystem read-only
[15019.549069] EXT4-fs error (device sda1) in ext4_da_write_begin:2533: IO failure
[22018.927076] readlink[14332]: segfault at 4 ip b77c2481 sp bfbe0f00 error 4 in ld-2.15.so[b77b7000+20000]
[22019.009226] dirname[14334]: segfault at 4 ip b776e481 sp bfbcb140 error 4 in ld-2.15.so[b7763000+20000]
[22019.086704] mkdir[14337]: segfault at 4 ip b7707481 sp bfc313a0 error 4 in ld-2.15.so[b76fc000+20000]
[25526.009125] dell_wmi: Received unknown WMI event (0x11)
[25550.643627] uname[14420]: segfault at 4 ip b772c481 sp bfef8260 error 4 in ld-2.15.so[b7721000+20000]

 By the way: one of my complaints was that there wasn't any warning, but I noticed when clearing down every window manually this morning that there was an alert box *near the bottom of the heap* announcing "readonly file system", which seemed a bit pointless.

The drive was in a strange state when I first booted the CD (fsck couldn't update flags, as in http://blog.laukien.com/en/software/ext4-ist-kaputt-the-needs_recovery-flag-is-set-please-run-e2fsck-before-clearing-the-has_journal-flag.html), so I powered off & on again.
Fsck was fine on reboot. I tried  fsck   -c -c -k ... but there were no more bad blocks. It doesn't look as though "discard" has much to do with the corruption.

After all that, let's assume it's the drive.

I've now moved to the my new SSD (Crucial M4 64Gb *and* I updated the firmware to the latest 010G), and I re-enabled "discard", but I've copied my original system across (ubuntu 12.04) since I'm trying not to change too much at once. Kernel is still  3.6.2-030602-generic.