I think this is a fairly unknown aspect to Grooveshark, but definitely something that everyone should know know about.
Grooveshark offer a paid subscription that gives you access to streaming on your mobile via an app, but if you'd rather a free option, then this is a good work around.
Html5.grooveshark.com gives you access to the entire Grooveshark music catalogue (far more than that of spotify) directly on your mobile browser. As it's html5, it will of course work on the iPhone/iPad.
No need to sign in or anything.
It's not perfect, but it's pretty awesome if you want quick access to a song that you don't have on your iPod.