Apple's 3G iPad models will come with two unique aspects: only unlocked, no-contract services: It's not surprising that Apple will have Wi-Fi only and Wi-Fi plus 3G variants of its new iPad mobile device. Rather, it's that Apple finally got its demands met about how consumers will control the relationship with cellular carriers.
The iPad will come with a micro SIM, a new tiny form factor for SIM in mobile devices that's not yet in real use, as far as I can tell. (I had never heard of it before today, even though it's a settled 3GPP format.) Steve Jobs said it will be simple to swap out SIMs from other carriers, so that the US version of the 3G iPad will "just work" in most cases outside the US. It won't be until June or July that Apple has carrier relationships for direct sales and data plans other than in America.
The unlocked iPad will be coupled with two data plan options from AT&T, neither of which requires a contract or (as far as I know so far) any cancellation penalty. AT&T has some services now that you can turn on or off on demand, such as navigation.
The 250 MB/mo. plan is $15/mo; the unlimited plan is $30/month. While you might scoff at 250 MB, the iPad will have the same limitations as the iPhone in terms of downloading and storing stuff over the Internet, so outside of purchasing movies, the biggest 3G drain will be streaming video. Because the iPhone OS doesn't support Flash, streaming video must all be embedded H.264 format or accessed via the YouTube app or other applications.
I'm calling the 250 MB/mo plan "your mother's plan," because it's most likely to appeal to people who won't be heavy 3G users, and will mostly use the device over Wi-Fi at home or at hotspots. However, they will want the flexibility of having 3G available wherever when they carry the device with them.
The iPad still is slated to have the disappointing pairing of UMTS for upload (384 Kbps) with HSDPA for download (ostensibly HSPA 7.2 as with the iPhone 3GS); this detail is noted on the Tech Specs page for the iPad. The iPad will likely be a heavier producing device, especially given that there's a camera connection kit (USB or SD card reader) that will let you suck photos directly into the iPad. These will sync with iPhoto when you return to a Mac (or through other means specified in iTunes on a Mac or under Windows), but uploading photos during a trip will certainly be desirable, and limited over 3G networks to the paltry 384 Kbps rate.
I should note, of course, that the iPad will have 802.11n support, but it's unknown to me yet whether this will be a single-stream radio, which would use less juice and thus be more sensible in a device intended to have a long battery life, or a two-stream 802.11n adapter, which will drain it faster. Apple uses USB for syncing large amounts of content, and doesn't provide over-the-air sync for anything directly. (You can use its MobileMe service to sync calendars and contacts.)
That means that the gating factor on most networks will be the Internet connection, not the wireless LAN. Having a 50 Mbps or so top rate with 802.11n single stream won't really be a clog on the iPad's abilities.