I upgraded to MT 4.1-OS just now. Aside from having to rebuild all my pages (and the tag cloud apparently getting semi-busted), I had a minor panic during the upgrade, getting this error:
undefined type: at lib/MT/Upgrade.pm line 1316
Fortunately, this forum post helped, but the actual lines you want in lib/MT/ObjectDriver/DDL.pm line 410 are:

}  elsif ($type == SQL_FLOAT) {
return 'float';

not SQLFLOAT, as Jayson says. (Watch your braces!) Looks like this is only really an issue with PostgreSQL, which might explain why this bug hasn't been fixed in a release version for well over a month.
I'd have posted a correction there, but I didn't feel like navigating their registration. (Probably I could use OpenID, but I keep forgetting my username and password...)
Aside from those minor quibbles, things seem to have gone ok, so, yay me.