The Cingular may be locked or unlocked. They are generally locked but can be unlocked online. If the phone is locked, you will need to get it unlocked first. To find out, stick a SIM card in (not the Cingular one) and see if you are able to get reception. If you get reception, then you should generally be ok but you may have to get the MMS and GPRS settings for your network provider. Just google O2 GPRS settings and you should be able to find it, for example. Once you know the phone is unlocked, you can use any SIM card other than 3.
Differences between a vanilla 680 ROM and the Cingular one will tend to be additional Cingular software and Blazer will be set to use Cingular's proxy server to access the web (you can change it)
There is a wealth of information at Treo Central with a forum dedicated to the 680. Visit
here
A couple of cool pieces of software to get a hold of are
Metro (a public transport journey planner for most of the major cities around the world) and TCPMP (multimedia player or you can use Real Player which has the added advantage of being able to keep playing music in the background after the app has been hidden but only plays mp3 and realaudio)
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