Will third party plugins designed for CS5 work with CS 5.5?
Hard to say, but I have to believe there won't be any differences. This isn't the major framework overhaul that CS4 to CS5 was (e.g. 32-bit to 64-bit) which necessitated rewriting all existing plug-ins. There may be compatibility updates from certain manufacturers if there are issues, but that would up to the manufacturer to handle that. I would suspect that any of the major plug-in writers have already had a beta version to test out with their products.
When in doubt, ask ProDad...
I believe plugins written for CS5 will definitely work in CS5.5.
In the AE forum Todd made a similar remark for plugins in AE.
In general, plug-ins written for CS5 will work for CS5.5. There may be individual exceptions, but not because of any across-the-board change.
I can confirm that Magic Bullet Looks and Vitascene works perfect with 5,5.