Is launching threads okay in a web app?

[David Wall]

I know that in the EJB model, it's not correct that beans launch their own
threads.

Is that also true for webapps in tomcat?  Is there a portability issue if I
have a servlet launch it's own thread for doing application-based
housekeeping chores, such as listening for new messages via JMS and
POP/IMAP, or periodically cleaning up "old" database entries, etc.?

Thanks,
David