It looks like such a good idea, that I thought it'd be nice to have that on my Palm, too. Candidates for this are WordComplete and TextPlus. They were reviewed head to head in this review from Nov 1999 (the review compares WordComplete 1.0 (currently 2.0), TextPlus 3.0 (currently 3.8) and LookDA (which the reviewer found more lacking))
Interestingly, I found these because the makers of the fitaly keyboard didn't allow them to be used in this contest which compared speed of input methods (fitaly won, of course). Gee, what are these products that I'm not allowed to use because they'll make me too fast? It looks like you can use fitaly along with either WordComplete or TextPlus for super speed.
Does anybody have experience with any of these products?