I have been using an Apple iPhone in Chennai soon after it's release. Airtel announced 1 year of free 500 mb per month GPRS / EDGE with it. But they will never tell you how they bill it. I used 47.7 MB (of my so-called 500MB) last month and was charged 500+ bucks extra.
So here is what you really need to know about Airtel GPRS Billing: The Billing Pulse is for every 100KB. So if you open MAPS and locate your current position it would be 100KB even if you have actually consumed 4KB. If you have set your MAIL to be checked every 15 minutes you are probably using up 100KB every 15 minutes. So that's about 10MB per day for every mail account. If you are using Chat, Browser or any other application that uses Internet or even location services Airtel is cunningly billing them.
While my actual usage was 47.70 MB, Airtel considered it far about 500MB and charged me mercilessly. Even tougher to follow up with their customer service to demand explanation. I wish we had better service providers.