Here is the updated bookmarklet: Phishy? (tested on firefox 2.0 only!)
If this does not work try turning debug to true above if you want to see the encoding.
Update: This still uses the GET method for checking the URL. Phishtank recommends using the POST interface (which will remove limitations on URL length: base64 inflates the length by 33%). Implementing that would need some kind of xmlhttprequest hackery. Stay tuned...
Update3: Thanks to "till" who commented below, here is the bookmarklet using the POST method so now the solution will also work for really long URLs. Till's solution is good, but it makes users trust his site (in addition to phishtank). So basically user has to trust that he is not trying to filter the results being presented..
I have also merged the two earlier bookmarklets so that the current site location will be autopopulated in the prompt, so that user can easily change it if he wants to check a URL different from the one he currently is on.