Yes, a Blackberry Bold 9700 will work in the US.
From the RIM website, spec page:
GSM: 1900/1800/900/850 MHz
As long as your handset will work in the 1900 MHz band, it will work in the US.
You will obviously need to make sure it's unlocked though.
Getting a data plan is much more difficult than getting a voice/SMS plan whilst you are in the US. In many cases you need a pay monthly plan in order to get data, which obviously you can't get not living there.
One suggestion, mentioned elsewhere is to get yourself a Virgin Mobile US MiFi router. See
here. This is what I'm going to try and get when I'm in SFO in a few weeks. $149 initial cost and $40 for 1 month unlimited data usage without needing a contract or credit check.
A great idea would be if someone at Virgin would make these available for sale on-board VS flights. I'd buy one without even thinking about it. As we have seen from here, there is a demand for such devices from US visitors and I'm sure they could easily sell them.
I also wish networks would start looking at their ridiculously expensive data roaming charges
outside Europe. There's no excuse or justification for such high charges other than for profiteering.