I think there was roughly a five month lag between PSE4 for Mac and Windows... (I think ~10/2005 vs. 03/2006 or so) They indicated to MacWorld at the time that they didn't place a high priority on simultaneous releases on Mac / Windows.
But also, it looks like mostly the new features in PSE5 are fairly foo-foo... 3.0 brought major changes, and 4.0 seems to have only a few additional features in terms of actual photo editing. I think a lot of the reason it's being so heavily developed at the mo' is that, on Windows, it does double duty, for the functions that iPhoto subsumes on Macs (and which Adobe generally seeks to complement rather than replace), and the traditional photoshop functions.