I just did the 2GB upgrade an hour ago. I use iStat to report CPU temperature in MacOS. It sat at ~52 average before the upgrade, and now it's sitting at ~54 or so.
I think in my case this probably has less to do with the new RAM causing extra heat, but with the removal and re-application of the thermal pads. The thermal pads were stuck to the metal cooling area on the bezel that you remove, so when I re-applied the bezel there could still be little pockets of air between the thermal pads and the processor. The best solution here would probably have been to buy two new thermal pads and re-apply them to the CPU itself.
As it stands now, I don't really care about the extra two degrees, and even that temperature is well within standard operating temperature pre-upgrade. There could easily be NO temperature difference, but I'm just reporting the first thing I saw after booting and the last thing I saw before installation.
So I think if you've got some temperature problems, the first thing you should check is that your thermal pads didn't tear during disassembly or reassembly.