A 3G phone is quite close to broadband for data rates, though I can't remember what they are exactly. You can use a 3G phone with a laptop via bluetooth and get good speed out of it.
3G is the equivalent of mobile broadband but it's nowhere near as fast (384Kbps I think the last time I looked).
You can also get an XDA 2 which has a built in wireless technology which will let you use it with your home broadband or at wireless hot spots : http://www.my-xda.com/comp.html
I believe that Nokia is about to release a mobile phone/device that has wi-fi built in so you could access the internet at broadband speeds that way. Of course you would have to be in range of a network and I think its meant to be used as a mini media device, eg web browsing. movie watching more then anything else.