Thinking about signing up for iPhone 3G service? Here are some pricing details you'll want to know about.

1. You have to pay $18 or $36. A one time $36 activation fee still applies to 3G plans, the way it did for the first generation iPhone. If you're a current AT&T subscriber, this fee is only $18.

2. Data costs more. On the original iPhone plans, unlimited data cost $20/month. The new iPhone 3G plans cost $30/month.

3. Exchange costs $15/month more beyond that. The normal 3G iPhone data plan costs $30/month. Enterprise data costs $45/month. This buys you access to all the standard Exchange features. If you need to get Exchange mail, or access data inside your firewall, you're an enterprise user. AT&T isn't saying they'll disable ActiveSync and Exchange on normal iPhone plans for non-corporate users so we're unsure how they'll police this policy. Our Christina speculates that the enterprise plan sets up the server to accept exchange support; you could do IMAP from your exchange account but if you want non-MobileMe push, you gotta be on the enterprise tier plan.

Follow the jumps for more plan pricing details.

4. SMS isn't included. 200 Text Messages a month start at $5 and go up to $15 for 1500 and $20 for unlimited SMS messages.

5. You can buy SMS a la cart. SMS messages cost $0.20 each if you aren't on one of the monthly plans. This is a bit pricey. Late last year, I was still paying $0.05 for AT&T SMS a la carte. Then they jumped to $0.15, and now $0.20.

6. Voice minutes still start at $40/month. There are no prepaid contract free Pay as You Go plans available for the 3G iPhone. $40/month buys you 450 minutes. Price plans go up from there to $200/month for 6000 minutes.

7. International plans are available. You must activate International Roaming before leaving the US and set up a plan. Call 866-331-0500 to find out more about international roaming and long distance. Keep in mind this consideration is true of nearly all smartphones from AT&T and not just the iPhone.

8. There are no 3G-less discounts. If you live in an area that doesn't have 3G service, you're still going to pay at least $30/month for data. AT&T no longer offers the original $20/month data plan. Many people consider the $20/month plan an incredible bargain.

This article was originally published on Tuaw.