Like Wingedwraith says I have tracked down the problem and reproduce to the same thing :
Using a non apple screensaver that crashed, made mission control, gestures and spaces unavailable.
The exception thrown in the crash report is this:
Date/Time: | 2012-01-19 10:16:21.446 +0100 |
OS Version: | Mac OS X 10.7.2 (11C74) |
Report Version: 9
Crashed Thread: 0 Dispatch queue: com.apple.main-thread
Exception Type: EXC_BAD_ACCESS (SIGSEGV)
Exception Codes: KERN_INVALID_ADDRESS at 0x0000000111f0c00c
VM Regions Near 0x111f0c00c:
MALLOC_LARGE | 0000000111e54000-0000000111f0c000 [ 736K] rw-/rwx SM=PRV |
-->
MALLOC_LARGE | 0000000111fcc000-000000011213c000 [ 1472K] rw-/rwx SM=PRV |
So the only way to make it work again is to kill the dock by typing in the terminal Killall Dock
Of course it can be avoided by using one of lion's screensavers