the update manager isn't picking up 11.04
By default, update-manager will only prompt you to upgrade to released versions of Ubuntu. As 11.04 has not officially been released yet, you were not prompted.
That said, if you want to upgrade anyway you can run the following command (from a terminal, or with Alt+F2):
This tells the update manager to check for new Ubuntu releases and include development releases in the search. This should give you the opportunity to upgrade to the 11.04 beta.
If you continue to apply updates as they come out, this will eventually give you the final 11.04 release when it is ready.
Ubuntu 11.04 is still in Beta (2nd Beta) and is due for release on 28th April, so until then, your update manager won't allow you to upgrade to the 11.04 (the reason being is unstable). You can upgrade to Ubuntu 11.04 from your maverick installation only through ISO of 11.04 (as it has option to upgrade during installation), but it is not recommended for production system, but it will be appreciated if you upgrade to unstable and report any bugs that you find, or you wait until 28th April to get fairly stable version of Ubuntu.