This will end in tears because it just isn't possible to keep the wallet URL secret.
Instawallet 2.0 all over again.
instawallet was killed purely with the google search [ site:instawallet.com ] which crawled all pages and listing all 'secret urls'. so far rushwallet have got good enough code so that it doesnt list all pages on google...
so show me another way to break the secret?
Browser extensions which may look at browser history, or any malware which finds browser history on disk. Using this on a foreign computer and not wiping history afterwards. Social bookmarking sites. Other cloud bookmarking/sync services. Social engineering to trick users into showing their URLs, because you know, URLs are safe.
Maybe it's not as easy as google hacking, but I am very confident people will find ways to attack that, even if just because browsing history and bookmarks are usually not that sensitive, not a worthy target, and thus most likely ill protected.