Well i'm no expert, but this site seems pretty acurate to me. Everything i've ever heard about breaking in engines all go along the same lines:
1. Don't push the thing too hard, which is why people say no faster the 60 mph, or 3500 rpm or whatever. The number is just sort of a standard so that you don't burn the thing too hard.
2. Vary the speed so that the engine doesnt' just do the same thing all the time. Varying pressures and things are better then just one, cause then things move around a little better or whatever.
3. Usually by the time you've drivin 500 miles varying the speed, but not pushing too hard (although once in a while a little rough) Things have pretty much done all the settling and seating their gonna do.
My opinion is that there is no "magic number" or anything, realisticly as long as you follow the basic idea it'll be ok.
Another word of advice for those rebuilding engines:
If your having someone else do the job for you, don't find the cheapest one you can, that's what my dad did, and now the car is seriously screwed. It presents me with the most mysterious problems.... (it is my daily driver) make sure you don't have some yahoo doing the job, because it'll end up costing you more down the road. Bite the bullet and make sure it gets done right. Sorry, i'm just a little bitter about this right now, because i have to get a new car.
anyway, good luck with the engine.
Oh yeah, be sure to change oil, filter all that good stuff after the break-in....