You're not alone, that's why there are a few shareware programs that function as GPRS monitors. They count the mb you've downloaded via GPRS (which is the high speed data portion of the T-Mobile service). You will not receive a warning from T-Mobile, or any other wireless provider, to my knowledge. An estimate is 1-10k per web page and emails are best judged by looking at your current email desktop program to see the size of the messages you receive. Simple text emails are usually under 5k, but if you receive attachments or complext html mail messages the number will be higher.
10 mb is generally good for light to moderate users. One way to save on mb charges is to turn off image loading in Pocket Internet Explorer when you don't need to see graphics on web sites.
-------------------- Lisa Gade Editor in Chief, MobileTechReview