You're right. I'm not sure if there's a way around this. Just got my phone about 2 weeks back, the version is 3.1.2. Jailbroke it using blackra1n, but when I restarted the phone, it requested for iTunes to be plugged in.

So I did, and the only option was a message to "restore" the phone. The next step required a 300MB "OS update" download via iTunes which took a few hours on our wonderful Streamyx line. No ways around it, so went ahead with the restoration process.

Lost some apps, smses and photos, but otherwise everything went OK.