"Sympathetic User"'s support encouraged me to actually fix this problem today. Turns out the problem with MythTV was with reliance on a bug in Qt <= 3.3.3 which was fixed in 3.3.4. The MythTV folks have checked a fix in, but it'll likely be a while before that shows up in portage, so here's what to do in the meantime to fix the problem on Gentoo:

mask Qt 3.3.4 (add a line with ">=x11-libs/qt-3.3.4" to /etc/portage/package.mask (you may need to create the file anew))

emerge qt (this takes a while, you'll have time to write a blog entry about your trials and tribulations, read a few articles on news sites, get yourself a drink, sit watching the compile for a few minutes, then give up on waiting and head off to bed, then realise you can sleep and get back up and realise it's done)

restart mythbackend (/etc/init.d/mythbackend restart) to have it pick up the new (old) Qt

drop your channel database (note for ATI TV Wonder Pro users, this will require re-setting every single channel's signal settings due to an error with the card (argh!)) using mythsetup (answer yes when it asks whether to eliminate the channel data)

setup a new video source (in mythsetup again) to use your datadirect username and password, then tell your television input connection to use this new video source, then exit mythsetup

run mythfilldatabase

restart mythfrontend and check that the program database is now properly aligned with reality.

That fixes the problem for me, but it makes Eric3 (the Python IDE) look very different (seems to have lost the 3D effects for buttons). Not sure whether it's going to be stable going forward... guess we'll see.

(Oh, this is all from a post to the mailing list, re-cast as Gentoo instructions)