Well, I got my Pronto yesterday, and immediately tried programming it with X-10. Sadly, it has the same problems/limitations as the RC-2000 Mk 1 in this regard.

I have been able to teach it all the number keys, the on and off for individual devices, all off (I haven't tried all on, but I suspect that it's not a problem), dim and bright. Herein lies the problem--the Pronto refuses to repeat the dim/bright controls when you hold down the key.
While in learning mode, I've tried pulsing the signal and a variety of other methods, but the best I can do is to have it memorize a medium length sequence (a fixed number of steps) of dim/bright.

So in order to go from full bright to full off using the learned dim function, I have to press the corresponding button on the Pronto at least three times; holding it down doesn't work as the Pronto will NOT repeat the IR code. Tried this with several Sony remotes with no problem, and the original X-10 IR 6-in-1 of course, had no problems. For some reason, the RC-2000 and the Pronto (same engineers?) won't "repeat" X-10 IR codes. Plus teaching it the codes is a trickey affair...