You might have to wait a few weeks, as PrPro CS5.5 will be released in May. Anyone, who did beta on it, cannot comment yet.
Adobe has always been good at configuring their programs, so that one can keep an older version, and have it coexist with a newer one. There is much to be said for that, as there are always some changes in how things are done, and if one gets a rush job, they might be much more comfortable with their "old friend."
Yes, I have both versions installed on the same computers. No problems.
As Todd says it is possible (we're doing it too), with one specific exception: you cannot co-install two versions of Acrobat.
Every other program in MC will co-install just fine, but you'll have to decide if you want Acrobat X from CS5.5. or Acrobat 9 from CS5.
Note however that as with any other co-install, the file registrations can get a bit confusing. The OS will see the new version as owning the files, so double-click a PPROJ in Windows Explorer and it'll open in Prem Pro CS5.5 - however Adobe Bridge is collection-aware, so if you double-click the same file in Bridge CS5 it'll open in Prem Pro CS5, and from bridge CS5.1 it'll open in Prem Pro CS5.5. The icons are the same, but if you get lost Prem will warn you, as opening the "wrong version" will trigger the import old project dialog box. Also your plugins from CS5 will need to be re-installed for CS5.5 as it runs in a different folder tree - they should work but it's not 100% guaranteed.
Since project files cannot be directly passed between CS5 and CS5.5, it's important to keep track of which you want to work in - and for that reason we've got into the habit of using Bridge as a file browser. You still have to be careful as there's no visual difference between the two copies of Bridge, and they share some preferences (for example if you make Bridge CS5's UI white, CS5.1's UI goes white too).