This problem seems to occur on a weekly basis.
There are elements of Lion taht become ustable and can cause several things ewither simultaneously or individually, including Mail crashing if you browse messages while new messages are downloading, Wireless connection being lost, Time Machine failing to prepare a backup, iTunes failing to open, activity monitor faling to open etc.
When the system becomes unstable like this it will generally be necessary to force a shutdown and Spotlight's index will be or has already become corrupted. When you restart it wil then need to index from scratch, slowing things down.
Running Disk utility to repair permissions, you will probably find that quite a few repairs are made.
For users of Fusion, it is worth noting that once Lion becomes ustable in this way there is a very high chance any open virtual machines will become corrupted when you shut them down to then shut down Lion. Make sure you are using Snapshots so you can revert to an earlier version when restarting. Similar precaustions are probably advisable for Parallels.
This is a mess.
I hope Apple are able to resolve the issue/s quickly.