For reasons yet unknown (although we have our suspicion...), Google is currently returning the RK value of zero (0) for any and all pages. This kind of breaks this tool in the way that it no longer displays Google internal, up-to-date PageRank, but you can still use it for checking Toolbar PR across datacenters.
In the words of Google Engineer Matt Cutts:
Q: "Is the RK parameter turned off, or should we expect to see it again?"
A: I wouldn’t expect to see the RK parameter have a non-zero value again.
Q: "What’s an RK parameter?"
A: It’s a parameter that you could see in a Google toolbar query. Some people outside of Google had speculated that it was live PageRank, that PageRank differed between Bigdaddy and the older infrastructure, etc.
Live PageRank now gives you the PageRank value for a lot of Google datacenters.