I have a Jailbroken 3g 16gb, I am waiting for o2 to allow its staff to upgrade to the iPhone4, we cant at the moment due to stock levels!
After using spirit to jailbreak my iPhone, I have found some many excellent apps from the cydia store that make the iPhone much more functional here are a few of my favourites.
RealSMS - this is a paid app, but well worth the price, excellent SMS app to replace the standard messages. This app allows incoming messages to be viewed in a text box right on the screen no need to exit what your doing. You can read and reply or call the sender right from the text box. This app alone almost takes care of the need to multi task as most f my requirement to do this revolved around texting, now I just receive a message, respond on screen and send, no need to interrupt what I am doing.
AnyRing - This allows you to change ring tones and SMS tones. You can use any tone from your itunes library, and set specific tones for specific contacts.
IBluetooth - Allows file sharing between Jailbroken iPhones, you can send pictures, tones and lots more.
IPico Movie - Allows you to stream some of the latest movies directly to your iPhone.
