True. I would port my line to Digi if the 3G coverage are extensive.
Then again depending on your usage it's hard to break the data usage limit. Well at least for me. Per month on average I will do about around 500 and I am on the 3GB plan I think with Maxis.
At office I will use normal messaging except for Whatsapp. I will turn on the rest on my IM+ my when I leave office and go for walkabout before coming back home. Only while doing the walkabout I will use the data alot more. Mails only personal email accounts in there. What else....well those telnet/ssh/rdm thingy only if i need to access for my work stuff if I'm without laptop
Depending on your lifestyle it can either make or break the limit there. If iphone is your main source of internet then yea, possibly. If you are working with normal net access and with net access at home as well then this should be no problem

ROFL...I got a limit of 6K and when it hit 6-7k maxis will be busy sending me sms and call to clear the current bill otherwise they would bar it although not hitting bill cycle yet.

Another phone just got the sms from maxis saying exceeding or going to exceed 500MB soon. Which I think it's true cos she's been using the phone for all her activity since I sold her laptop cos too ld edi. So it's one single net access for her for now. She's using it for twitter, fb, email, surf, whatsapp, im but no youtube. Updating/buying apps as well

Mine pretty much alive so those service are still there. guess you can call the custsvc
maybe they remove you from the service (highly unlikely but can happen)
doesn't met those requirement (they increase in cc limit so you never exceed the new one more than 80% etc)
bug (most common..hehe)

Prolly cos mine was since more than 10 years ago and this case loyalty doesn't pay..haha
I think I lost the svc for short before it came out of the blue again but for maxis I don't use auto debit, only digi. i rarely call custscv and if i do i normally will instruct them to check this and that or talk and escalate to the engineer. that is if the problem been lingering for long
good thing on maxis i don't have to queue for iphone when everyone was waiting for 2-3months back then. just call the custsvc, argued abit, asked to check my track record etc, out of the blue, "you can go to klcc, they will have the iphone for you now" cos i need to fly out the next day. for once i think loyalty pays for maxis..lol

I would highly suspect the possible culprit to be the apps itself among others with the youtube taking your prorated quota. Let me just show you few example here based on what I have in my apps.
Awesome Note Lite (free): 8.4 MB
Awesome Note (paid): 10.3 MB

Angry Birds Lite (free): 10.2 MB
Angry Birds (paid): 13.3 MB

Twitter: 4.5MB
Tweetdeck: 2.4MB

As you can see, paid or free doesn't matter anyway since if paid it will go to the iTunes but what matters would be between you and telco in this case if mostly are downloaded using cell line. I can understand that after getting the iPhone you will start downloading stuff from iTunes. Well iPhone have this magnet that can have everyone go nuts as soon as they get their hands on it with all those apps.

MMS unlike SMS runs on data network. As such if you turn it off on your setting your won't incur any data usage at all and those that need data to run will be put on hold.
Problem with MMS unlike SMS are much more bigger. Unlike SMS which are very light and run's on voice network is a very matured product, MMS is not like that. It is true that MMS will be on pending mode but depending on which telco you are using and what technology employed, don't expect the MMS will come to your inbox the moment you turn on data. It might take a little or quite long while for that to be delivered. It doesn't have the constant retry feature as SMS.
If you know you will receive MMS nightly you might need to turn it on for longer period that you might expected. Be very careful as well as all other applications that might use data, will take this "open data window period" to do some other activities in the background as well.