So I got sick of setting up a mailbox every time someone wanted to send as an email address. I checked into it and it turns out setting send-as rights on a distribution lists is really easy. If you are running Exchange 2007 just open up the exchange shell and type:
|get-distributiongroup groupname||add-adpermission -user username -extendedrights Send-As|
Props to this blog for putting me in the know.