As noted elsewhere, OS X Lion can still be purchased via a call to 1-800-MY-APPLE. $19.99 + tax.
As far as I know, this article is still correct on the likely but undocumented policy of security fixes. Another reason I want to upgrade is to be able to test software on a later OS X level.
Throw in another $50.00 for the latest VMware Fusion (I had 2.0.6 previously, which doesn't support Lion) and a lot of wasted time fiddling with Xcode and Mac Ports to restore a working build environment (the upgrade wiped out symlinks which were part of the old Xcode (as well as the guts of Mercurial), new Xcode doesn't provide autoconf, ...). I guess everybody else went through this a year ago.