Junk mail filter for iPhone via imap
The best idea is to pay for (or set up) server side junk mail filtering. That allows all the clients to only spend time downloading non-junk.

Have a look at Mail Route (10 percent off link from the podcast where I heard about it sponsoring most recently) - http://mailroute.net/thetalkshow.html

You probably will spend more on the service than the cost to run your Mac all the time, but the filters will likely be much better on the hosted site than OS X mail client.