While this wouldn't explain the descrepency between your PCs and Macs, the Chicago area has been a little dodgy lately as they go through preparations for the speed upgrade, which one tech support person told me could happen tomorrow (I'm not holding my breath). My upload has been fantastic, always in the 42 - 45 KB/sec. range, but my d/l hasn't been above 3200 - 3300kbps in a while.
Of course, speed tests aren't always reliable (how many times have we been told this), but they are a pretty good indication of what's happening on the network. A more reliable indication would be real-world download speeds. For example, d/l a large file from a fast server. I d/l Firefox nightly builds a couple times a week. In these instances, I've been seeing speeds up to about 450KB/sec. which is right at 3600k.
So why the difference with the PCs? Not sure, but it could have something to do with the way Windows caches java applets differently than OSX, but I can't say for sure. Apple also makes their own modifications to Sun's Java, which could also account for the perceived (and real) slowdown.
