Will Ubuntu 11.04 come along with up to date Gnome-Shell packages?
Unfortunately it seems highly likely that GNOME Shell will not make it into the 11.04 repositories. This is because of its dependencies on the GNOME 3 stack, which won't make it into 11.04.
There is likely to be a PPA with these underlying GNOME 3 libraries and a "stable" version of GNOME Shell. GNOME 3 is due for release on April 4th, so around this time I'd start to keep a look out for this PPA.
For more details see the mailing list post Status update on GNOME3 and natty