One of my customers wants some fancy e-mail (collaboration, Outlook, calendar sharing push-mail etc)stuff and I have recommended Google Apps with their gmail client. They have a quite smart pilot system with dual-delivery of e-mail which makes it possible to have both the Plesk mail for the current users while the pilot users try out the new platform. There are two ways to implement this. On is to point all mail too Google and let them forward it to the old server. Not very good for a pilot in my mind. The other way is to let Plesk/qmail forward the mail for the domain to Google and deliver it to the Plesk mail server. The problem is that I really don't know how to do this. I have checked out a few blogs and FAQs about qmail but as soon as virtual domains come in they get a bit fuzzy.

Anyone have any suggestions of on how to copy all incoming mail for one domain to another server as well as to the current server?

Have you tried smtproutes? Example:

cd /var/qmail/control/

#cat smtproutes
example.com:1.2.3.4

have you tried this:
http://www.thedotproduct.org/2010/04/domain-forwarding-email-with-qmai/

Thanks a lot. Seems to be working great!