How does one downgrade TextMate Bundles?
I'm also running Textmate 2.0 alpha.9547 so am quite interested in hearing which of the bundle updates caused the problem!

But to your problem, one simple way would be to restore the offending bundle from your backups.

On my system I find the bundles in:

/Users/[username]/Library/Application Support/TextMate/Managed/Bundles


The harder method would be to locate the download site for the bundle and grab an older copy.  The textmate bundle repository is at: https://github.com/textmate/

To find a particular bundle, just append the bundle name, including the file extension to this path.  For example, the TODO bundle source is at https://github.com/textmate/TODO.tmbundle

From there you would need to clone the git repository and then use git commands to checkout the older version of the source.

I do see that there appears to be a global change to add a "ruby1.8 shim" 6 days ago and bundles have been modified as you noted. I did some quick checking and I appear to be running with the same modifications and as far as I can tell so far, the bundles are working OK.

(would have done this as a comment to the question but I don't have quite enough reputation yet.  ;)