AT&T SIM card work with Cingular phone?

This is probably a dumb question, but I am currently looking at getting a used Nokia 6820 off of Ebay and Im wondering: now that AT&T and Cingular have merged, will my AT&T SIM card work with a Cingular phone? Also, if I get the phone unlocked, will ALL features work with the ATT network (ie messaging, web, etc) or just voice?

Thanks everybody!

Best,
Neal

I'm going through a similar situation right now, and in short, the answer is no. I have an AT&T phone (6820) and SIM card, and I'm "migrating" over to Cingular. I've been waiting over a month for them to give me the unlock code so I can use a Cingular SIM in my phone, and still no luck. It was the guy at the phone shop that told me there were, um, other ways of unlocking my phone. If I have any luck, I'll let you know.

guys, post your imei, service provider, to www.kisland.com i can give you the code there. were not allowed to post any unlocking code to this site which i respect that.
click on computer forum and click on gsm forum. i'll see you there.

Thanks erwin, but I got it already from the unlockme.co.uk forums. A great site, use it if erwin's doesn't work for ya!!