It's so long since I've had to do an A&I I can't actually remember but
http://support.apple.com/kb/HT1710 suggests that all of /Users is moved to the new installation which would include mailboxes, contacts etc. It would be a good idea to have a separate back-up of (at least) your 'Home' folder and a note of your e-mail passwords.
If there was a power cut/HD died/optical drive died during the A&I you
could lose everything. Not likely but possible
Good luck,
t t