In the installer's partitioner, if you change an existing partition (say, NTFS) to ext3 so that you can install Ubuntu there but make no other changes (e.g. do not resize any partitions), partman mistakenly does not record the partition table as changed, and so the partition type field in the partition table remains as HPFS/NTFS. This causes GRUB to fail to install because it's fussy about the partition type.
I believe this is a fairly common reason for installation failures, and so I think it's worth backporting this fix to Ubuntu 8.04.2.
This bug was fixed for Ubuntu 8.10 as follows:
... and backported to hardy-proposed here:
TEST CASE: Install Windows (for example, taking the whole disk), then start the Ubuntu installer and use manual partitioning. Be careful not to create, delete, or resize any partitions; instead, just edit the existing NTFS partition, set it to "Use as: ext3", and mount it on /. Finish partitioning, ignoring the "no swap" warning. Before this fix, GRUB should fail to install towards the end of installation; afterwards, it should succeed.
At present I know of no plausible regressions likely to happen with this patch, other than the partitioner breaking completely due to some kind of miscompilation. It's probably worth testing an LVM installation on general principles.