Would you consider creating a namecoin version of instawallet?
I think Namecoin is a very interesting project, but would prefer to focus my resources just on the Bitcoin side of things for now.
Is it possible for someone to generate addresses locally and inputting them to your site, sort of like a brute force way to find random wallets? I noticed the url is different from the wallet address, which is good, but I would still be concerned about an attack like that.
Knowing the Bitcoin address of a wallet doesn't improve your chances of guessing an Instawallet, if that's what you mean. As to randomly trying Instawallet URLs: the search space is big enough, that this won't get you anywhere.
@JAV, You may also have to worry about Toolbar programs (like Alexa toolbar, Ask toolbar, Bing tool bar, Yahoo Tool Bar, Google Toolbar, lots of Firefox plugins). I believe that some of these send URLs back to the "mother ship" to help with page rankings and site analytics.
Thx for the heads up, but how do you propose I should deal with them? It seems to me, that if people want to send their private data to a cloud service, it's up to them whether they trust that provider. I'm not the only service that uses secret URLs. You can, for example, create YouTube videos that can only be accessed through a private link. As far as I know, these services also don't deal specifically with toolbars. But I will mention it in the upcoming FAQ.
Could you implement some kind of thing like Mt. Gox's API where I could send BTC by just opening a URL, and then as long as enough BTC were in my wallet, it would send them.
I have been toying with the idea of providing an API. It will probably happen at some point, but I can't promise anything right now, there are still lots of other things in the queue.