There is no rootkit
Now, I don't like this SecuROM stuff that comes with BioShock any more than anyone else, but the truth is that it is NOT a rootkit. Just because something shows up in RootkitRevealer doesn't mean a rootkit is installed on your PC.
RootkitRevealer will show any Windows registry entries that have been hidden or made unaccessible using embedded or trailing NULL characters. This is because real rootkits often misuse this technique for their own evil ends.
A registry key alone without other supporting entries shown, such as hidden drivers (.sys files) or executables (.com, .exe files) are not rootkits. Obviously, if you don't know this, it's easy to assume that a rootkit is hiding on when something appears in the RootkitRevealer results panel.
All SecuROM is doing in this instance is preventing the key from being deleted/messed around with. Whilst it may look sinister, it is ultimately harmless (other than you can't see what's in there or remove the entry).
I don't support this SecuROM protection, but I just thought I'd at least help to allay some of the fears about running this game. I guess it just irks me when there's loads of people running around screaming "rootkit!" when there really isn't one.
By all means, avoid the game if you don't like the SecuROM protection, but don't avoid it because of supposed rootkits. There aren't any.
Hope all my babble helps someone. I'm heading back to Rapture...