I was reading an article by Matt Blumberg this afternoon on the topic of testing. In this article, Matt suggests that you could also use pay-per-click listings (like Google Adwords) to test subject lines.
Here's how: Create 5-10 subject lines for the same offer and list them all simultaneously on a search engine, using the same search term. Then simply track which one gets the most clicks. Usually, the phrase that wins in a search will be the most effective in an email.
The article offers more tips and tricks for testing and is actually excerpted from the book Sign Me Up! A Marketer's Guide to Creating Email Newsletters That Build Relationships and Boost Sales.