Short answer: nope.
If you are editing in CS6 it will auto save as CS6. Its not downwards compatible.
Best is to finish in 5.5 if you have to wait for the effects.
Which effects do you own for 5.5?
I had a Magic Bullet Looks effect applied and then the PP crashed. The problem was it was saved as a CS6 project even though I was editing in CS5.5 and unfortunately I lost some work when I re-opened the CS5.5 project.
The complete MB Looks Suite can be easily transferred to CS6 and works without problems. The same with all Pixelan plug-ins and Color Finesse. The only plug-in that does not work nicely is Denoiser II.
First of all, you may upgrade all Red Giant suites from CS5.5 to CS6 for free.
Secondly, similar issue has been discussed here. So, if you like experimenting, read that thread up to the end and play with options discussed (personally I like changing 'Version' value in XML document most). But bear in mind: everything is at your own peril.
Thank you all for that, it's good to know about the plugins. I'm still curious as to why a CS5.5 project is being autosaved as CS6?