To begin with, not to happy about the download only OS X Lion upgrade. Now when I reinstall I have to download 3.5gb to upgrade to Lion. I can understand Apple protecting them from someone buying one DVD and installing on multiple systems, but there are better ways to go about that.
Anyway, if you’re experiencing an overheating Mac system it appears that the new OS requires a complete reindex of your system. This can take awhile to run and will consume a lot of your system resources. If you look at your Activity Monitor you’ll probably see mdworker using anywhere from 5 to 90% of the processor. My reindexing is still going on but my system temperature should drop once the indexing is done (my system temp reached 168 degrees).