Rules, Rules, Rules... Redux

So, I've been reading Keep on the Shadowfell and eagerly anticipating 4e, and this has got me thinking about rules again. Basically, I have plans to run three different campaigns at this point (eventually I want to play again too, so somebody else better think about DM'ing in the future). Those three being Ptolus, a Pathfinder Campaign, and a 4e campaign. Obviously, the 4e campaign will be run under the 4e rules (redundant much?), but that leaves the question of the other two open. The Pathfinder rules and campaign setting will be coming out in the next year-- I like to think of it as 3.75-- so the Pathfinder campaign will be run under those rules. Ptolus was written for 3.0-3.5, but I think without much tweaking it can easily be run under the Pathfinder rules. However, I was thinking that there are things that I already like about 4e which may or may not be feasible to incorporate into house rules; sort of a 3.8-3.9 rules set (think at-will Magic Missile). But in my mind, one of the most exciting decisions I've made is that I'm going to run the 4e game in a homebrew campaign setting--within the parameters of the 4e rules. I haven't thought too much about the details yet, but that should wait until I get a look at the complete 4e rules.