When I wrote this: http://blog.fuzzierlogic.com/archives/425, the historical stats of SwissProt/TrEMBL weren't very publicly accessible. This is a situation that has, if anything, got worse. In my post I link to a page that at least had the sizes of SwissProt for old releases, but even that has now disappeared with the redesign of the ExPASy homepages.
In the tarball that you download for old releases (eg, release 2011_08.tgz, 6.9GB), there is apparently a docs/ dir, it may be that the info you need is in there somewhere. It does seem crazy to download nearly 7GB of data to be able to find this simple figure though.
EDIT - Yes, the docs/ dir contains the speclist.txt (same in current release), which I think has the information you're after.
I read this question and soon you will get better release notes for uniprot.org as well as historical ones. You can find recent release notes on the this beta page if you have any comments or suggestions please write to firstname.lastname@example.org.
The number of species is not really in the current release notes. The closest thing is number of distinct taxonomy identifiers.