I have just successfully completed a Database attached upgrade from WSS 3.0 to SPS 2010.
I used the Mount -spcontentdatabase command with the -updateuserexperience option.
The site is working fine. However, the look and feel is exactly the same as WSS 3.0.
The only difference seems to be that now Excel services is used when opening spreadsheets.
I can only select themes etc from WSS 3.0.
Any ideas how to change this? kind of defeats the purpose of upgrading if there is only 1 extra feature.