I just finished building a FC3/Dovecot/Sendmail/DSpam box. I have all the mail put into the home direcories in IMAP format. The only thing I had to add to was the following line in /etc/procmailrc.
DEFAULT=$HOME/Maildir/
The mail in /var/spool/mail is irrelevant after that; sendmail uses procmail for LDA, and it puts it in the user's home directories. If you want to recover the mail in /var/spool I suggest using the mb2md script. I wrote a little wrapper to do some sanity checking and to process large #'s, but otherwise it works great.
John
default_mail_env = mbox:~/mail:INBOX=/var/spool/mail/%u
One way of getting sendmail to deliver to Maildir folders, is to configure procmail as your delivery agent.
I suspect that most linux distributions have sendmail configured to use procmail for delivery out of the box.