gotcha: Arcmap 10 "save a copy" will create a 9.3 .mxd. If however you open a 9.3 mxd in 10 and then save it using the toolbar button or [ctrl]-[s] it upgrades it to 10 format, rendering it inaccessible to 9.3 clients.
Futhermore, save-a-copy will not let you overwrite the open map, therefore forcing a new document to be created. One must clean things up afterwards, after closing Arcmap, by manually deleting the old copy and renaming the new one. Needless to say most people will not be motivated to do this and there will be an escalation in the number of duplicate maps. yech.
This makes working in mixed environment very difficult. It would be much better if Arcmap saved the .mxd using the same version the file was in when it was opened. There should be a small noticeable but unobstrusive message reminding the user the file was "saved in 9.3 format", for example.
It would also be useful if ArcCatalog [select mxd] > properties showed the version number.