Today YouTube announced a mobile version of YouTube which appears to be a Java midlet that works with Sony Ericssons's K800. I'm surprised they didn't officially throw k790a into the mix as well, as the phones are very closely related.
It looks like the unsigned Nokia version will work with the K790a: k790a YouTube App
The signed version should work for Nokia's (N73, N95, E65, 6110, 6120 officially): Nokia's YouTube App
I posted on the Esato board as I was trying to figure this out. Let me know if you find anything else about this topic --