Are you sure "Note that the Last.fm terms of use don't allow streaming music to mobile phones, so you are not allowed to use Last.fm in a Nokia N900" is still valid?
I was using Mobbler, http://code.google.com/p/mobbler/ on my i8910 before I got the N900 for the last 9 months, and it has no issues streaming from Last.fm You need a paid account outside of the UK here in Europe, but Mobbler is still in working just fine.
Do they have a special agreement or do they get around this in a different way?

Stable and does exactly what it is supposed to do with minimal fuss. Regularly in use for 8-10 hours per day with no issues whatsoever...