Sid?
Lenny?
Etch?
Sarge?
Woody?
During these troubled times after a freeze is thawed, I never use the "update manager" unless it only wants to install or upgrade things.
If it wants to remove them... I drop to a command line and run this:
apt-get update && apt-get -u upgrade
Once that completes without error
apt-get update && apt-get -u dist-upgrade
and see what its "holding back" or "removing" or both. Thereby eliminating any problems before its too late.
Remember "upgrade" *ONLY* installs or upgrades packages, never removes them. The "dist-upgrade" or "smart upgrade" does. Breakage can occur that way.
Transitions from older versions of Gnome to Newer versions is fraught with difficulty when tracking testing or Sid
You do know Lenny was released right?
I'd also wait a 5-10 days up to 3 weeks for a dist-upgrade, unless you are willing to pick and choose what is upgraded.