i am very reluctant as to how the af of a rebel compares to the one of the k20d. i think people imagine the af system is the same across the range but i think you might be surprised. i haven't used a canon extensively, so i cannot comment, but i think many people would be surprised at how unimpressive anything bellow the 40d or so would be. other than that, if it comes down to the rebel against the k20d, i honestly think there's no contest, it's really unfair to compare them, the k20d is in a different league. play with them and decide for yourself.
please note, however, that pentax lens prices have been up by leaps and bounds lately, especially in europe, and probably us will follow. nobody knows why and what comes next, but the current situation looks gloomy (to me at least). the k20d is a great camera for the money, but i cannot recommend it wholeheartedly without mentioning these pricing issues. you might be better off with a secodn hand 40d, as heather suggested (i cannot believe i am saying that
), as the pricing for canon glass has been much more stable so far. if all you will ever need is the kit lens, the kit tele-zoom (50-200 or 55-300) and the fast fifty, you needn't worry about that, though; but it's never as simple, (d)slr's are all about lenses (?)
good luck, and, as somebody mentioned, either will take great pictures, don't worry about it too much