Well, Lion is out. I'm being cautious and haven't upgraded yet. Nor did I get Xcode 4.0 when I had the chance, to get a head start (now 4.1 is available and free, but Lion only). I plan to give it a week to watch reports of experiences and look into replacements and upgrades to important applications.
Theoretically, my current packages should work on Lion, but I've already had a report of Postgres not working, but I'm not surprised there. Please be cautious when trying them on Lion, especially the background non-application stuff (Postgres, PHP, Mapserver).
The frameworks and their utilities should work.
QGIS may have problems, or not work at all. Qt works with a lot of system-level stuff, so things can easily break in a major system upgrade.
As mentioned, Postgres doesn't work. The postgres user doesn't get created, which is critical for Postgres to run.
GRASS should work, at least the command-line stuff. The GUI may break if the new Python gets in the way.
PHP - no idea. The current version of Apache is still 2.2.x, so unless Apple messed with the organization of Apache, PHP may work.
MapServer - also depends on Apache.