If you or your organization are on OSX/Macs and are using Communicator for MAC i would strongly advice on not updating to Lion. since it crashes when u send or recive IMs & calls
Reports on twitter says that a reinstallation of communicator could solve it, but it did not put the error to end for me.
So Lion or Communicator. your choice for now.
Update:
tmgbabul Babul A. Mukherjee
@itommyclarke FYI The RTM release of communicator Mac works fine (13.0.0) on Lion, incl Ent Voice. Other vers crash.
Update 2: There is now an update released that fixes the problem so just check for updates and it should download and all is back to normal again.