I've dove into the forums and saw that 1.5.1 fixed memory issues, but a client wants to see release notes and I can't find them. Is there a blog post that disappeared?
I had a look around, but there doesn't seem to be any official blog post on 1.5.1. I also looked in the readme.txt file in both the standard zip and svn (http://svn.deconcept.com/swfobject/trunk/readme.txt), but they don't specifically mention 1.5.1 - Although there is a changelog there)
You are also correct in that the current download appears to include 1.5 again rather than 1.5.1. This issue had been raised (and fixed) some time ago, but now seems to have reared its head again...I'll contact Geoff about this.
In regards to the differences between 1.1 and 1.5.1, it was as you say to do with memory clean up on some F9 files (and flv streaming). Basically the method wasn't getting called in every instance, so it was moved. Here is a diff report I generated if you want to get nerdy :) http://www.misterhee.com/tests/swfobject.1.5v1.5.1.full.diff.html