The changelog for MobileSubstrate is always the same:
implemented critical fixes to the instruction decoders based on bug reports from people writing extensions
The timing of the releases tend to correlate with major new releases from either myself, Apple, or third party extension vendors. This is because those are the times when new bugs are detected by extension developers.
There is simply /not/ a changelog that would make any sense for MobileSubstrate to any end-user or end-developer. I can't even predict what all would be fixed by new releases of MS, all I can say is "omg, I can't believe that previous version worked, that was a /critical/ fix".
In the two or three cases that there has ever, in the history of MS, been a change that is worthy of a changelog mention, it is typically the following:
due to reports from users over the course of the last hour, I have made a subtle change to the loading mechanism of MobileSubstrate to improve safety and reduce bug reports
This occurred today, and was mentioned explicitly on my Twitter feed with a mention of what I did (CommCenter SafeMode).