I own a Hummingbird, and run it mostly as a 26″ bike with 120mm Maguras on the front. I do use it fairly frequently with a 29″ front wheel and the supplied rigid fork, and it does fly like a fast thing. A couple of things; as above, you may want to get a custom-designed 69er – the Carver, Trek and Singular Hummingbird seem to do this pretty well.
The only downsides I’ve found are that the front wheel rolls far more easily over things than the rear, which can lead to the rear hooking up every now and then. However, it’s a ‘first ten minutes of the first ride’ sort of thing – once you understand it, you can militate against it.
Second thing? I had to try a few 29er tyres before I found one that suited me and the tyre on the back of my bike – a 2.1 Advantage. Bizarrely, it was Kenda Nevagal, which I never got on with as a 26″ tyre. I switch between that and a Racing Ralph. It does *feel* like you get a lot more grip out of a 29″ tyre.
Overall? First time I rode it, I overtook someone on a FS bike down Park Life on Hombury Hill. Rigid, with one gear. 😀
*edit* Oh, and Singular Sam has some large sized Hummingbirds still going, discounted to silly money.