If you create the forwarder but not an actual account, email will forward straight through and you won't have to clean up an account after it. I don't believe it will get checked by Spam Assassin but Gmail does have excellent spam handling.

Keep in mind that forwarding email can get our server blacklisted as our server will be seen as the last place the spam came from. If the user marks email as spam at Gmail, he will be reporting Lunarpages, not the real spammer.

I was trying to figure out a way to just create the email address and forward the email to the new address and then create a cron job that would log into the email account and delete the emails that have been forwarded.  I believe this would eliminate the blacklist problem and automatically delete the emails.  I haven't figured out how to do it though.

It seems like Google email rarely gets blocked and I don't really see as many instances of Google email being blocked by many major service providers.  Setting up MX records to have Google handle your email may be a work-around.  The shared Lunarpages servers can sometimes be blacklisted by major providers as a spam source because there are many other users using the same shared IP address that could cause the blacklisting for everyone on the server.  When a particular user continually gets a server IP blacklisted, their email services may be temporarily suspended until the issue is resolved.

Here is a link to the MX records you would normally need to setup to work with Google:
http://www.google.com/support/a/bin/answer.py?answer=33352

You would need to have this setup with them before making any MX record changes.