passing the data limit on the iphone 4 warning?

Only if your carrier supplies such an app, as no third party app(including your carrier app, if available) would have access to the private API's necessary to supply such data. You can monitor data usage directly on your phone. Some carriers will send out alert emails/sms-AT&T in the states does that.

Quoted from AT&T:

AT&T outlines several different ways to achieve this:
* Customer Text Notifications On Data Usage. When customers begin to approach their monthly usage limit, AT&T will send three text notifications – after they reach 65 percent, 90 percent and 100 percent of the threshold. Customers will also be sent emails if AT&T has their email address.
* Data Usage Monitoring. Additionally, customers with iPhones and other select devices can use the free AT&T myWireless application to check data usage. The application is available for download in several smartphone app stores. Also, AT&T customers can call *DATA# from their wireless phone to check their data usage for the current billing period. They receive a free text message with their usage information. Customers can also go online to www.att.com/wireless to see all of their usage information, past and present.
* Data Calculator. The easy-to-use online data calculator enables customers to estimate their mobile data usage and get a better sense for which data plan is right for them. It’s located at www.att.com/datacalculator.

Not for your minutes, for your data which is completely separate from calling minutes.

The data Usage made available on the iPhone is not 100% accurate. AT&T has a free app at the iTunes app store called myWireless which is tied direct to your AT&T account. You can track your minutes usage - anytime/night and weekends/mobile to mobile, messaging usage, and data usage along with how many days remain in your billing cycle which is tied direct to your account with AT&T which is more accurate than the usage indicator on your iPhone which you also have to remember to reset on the day of your billing cycle. You can also view your full monthly bill in full and pay your bill with the app. The app also includes push notifications - Sounds/Alerts/Badges.

Give the app a try - it is free.