900Mhz cell phone on Cingulars 850Mhz?

I am looking to buy a new phone and i am interesting on buying a phone that is tri-band with GSM 900/1800/1900. Would this phone work on Cingulars network? i understand that Cingular works on the 850Mhz and 1900mhz but would the phone work without the ability to use Cingulars 850mhz? I live in the Bay Area in California and Cingular claims to have the best service here with the lowest dropped class and so on. I looked on the gsmworld website and it says Cingular's 1900Mhz covers my whole area and more. Note: I am still under At&t's contract but since At&t and Cingular have merged i don't think its an issue...right? So do i need to worry about the cell phone difference from Cingular's Service? If it does work is the call quality affected in any way?
Thanks a lot!

It will work, but you will only be using the 1900 mHz band as you already know, if you reach a region where Cingular is runing on the 850 mHz band, you will not have any coverage, not good if you're travelling, also the 850 mHz band penetrates building better than the 1900 mHz so again you might have signal problems inside buildings as well.
A note on the 900 mHz band, despite the close number, GSM 850 is not compatible with GSM 900. A phone that only has GSM 850 cannot work on a GSM 900 network, nor vice-versa.

I would get a quadband phone, then you are covered. The issue that may arise however, if you want to stay with your ATTWS plan is having the phone unlocked. I am not sure if Cingular will unlock a Cingular branded phone for you to use on the blue side.

I'm assuming babyjesus is not looking at Cingular/A&TT branded cell phones, hence the question about frequency bands, so if that's the case, most unbranded handsets come already unlocked so that part may not be an issue... no matter what handset he/she gets, unless it's an A&TT branded one it needs to be unlocked, Cingular will not do it, they will force you onto a Cingular account and make you buy one of their handsets before they unlock that cell phone to be used on your AT&T account.

If you are simply looking to save money on a new phone and will only use it in the US, you can get an inexpensive 850/1900 only GSM phone that will give you all the compatability you need.

I don't think the issue here is money, probably looking at a handset that A&TT or Cingular doesn't carry, you can get an inexpensive handset even if it's quadband.

lol...define the parameters of inexpensive.
The reasons I lean towards quadband phones when people are looking to upgrade or talk about tribands are:from what I read recently, most manufacturers are moving towards quadbands, and they allow for the greatest possibility to meet the frequency requirements should one find themselves traveling outside of their home area whether it be within national borders or outside of them. Essentially, it allows for contingencies we may encounter yet not consider when purchasing a phone to meet our immediate and initial needs.