Yes, I noticed the F8 functionality disappeared too.  You can use F8 to make it appear if it is currently closed, but you can not use F8 to make it disappear if it is currently open.

Did you close Object Explorer first?  Where is the focus when you try hitting F8 (after you've closed Object Explorer)?  Are you on x86 or x64?

I am on Vista x86.  It opens with F8 when it is closed, but you can't toggle.  I will try it later on Server 2008 x64 and Server 2003 x86, and see if there are any differences in behavior.

I spotted a couple of Connect items about this.  Here is one that seems like your issue, though Microsoft couldn't reproduce it:

https://connect.microsoft.com/SQLServer/feedback/ViewFeedback.aspx?FeedbackID=351585

And here is one that explains my issue (F8 has become an on-only switch, instead of an on-off toggle).  They were aware of it in 2006, and put it on the list for Katmai, but it wasn't addressed.

https://connect.microsoft.com/SQLServer/feedback/ViewFeedback.aspx?FeedbackID=126902

Maybe it is the VPC that is affecting it, not the operating system.  I am not using virtualization for any of the instances I've installed thus far.  (And maybe the other person who had this problem in RC0 was also experiencing this issue inside a virtual machine.)

As a workaround to F8 not acting like a toggle for Object Browser window, you can close the Object Browser window using only the keyboard by hitting F8 to give it focus and Shift-Esc to close it.