Would it be possible to leverage the ability of major community stake holders (Mt. Gox, Pool Operators) to incentivize or encourage activity on the testnet for new builds?
I think something along these lines is a good way to ensure proper testing.
The use of a reward type system would be ideal.
For example, a set reward amount, to be awarded by private exchange or business owners, for discovering a bug.
If I were to test the code and get a reward from the community for each bug discovered, I would be much more motivated to do so. The higher the reward, the higher the incentive for bug testing.
Another way of funding: Say in the client, put in an option to auto-contribute a tiny portion of each transaction to a bug testing reward pool that people may chose to opt into.
Something else to consider: a bug testing reward tracking website that people could also contribute to? Perhaps funded by auto-contributions from the client?