I finished my fitting-a-straight-line document; I will submit it to arXiv tomorrow. In the afternoon, Tsalmantza and I found that the non-negative updates are not only very successful, they are also very fast, much faster than the unconstrained updates. We think this is just because we are doing the unconstrained updates in a silly way (for R anyway). So it is on the to-do list to speed up the normal updates.
[after the fact: The fitting-a-line document is http://arxiv.org/abs/1008.4686. Thanks to everyone who helped out!]