The iPad will be available in two major categories. The cheaper of the two equips the iPad with Wi-Fi only, so you'll need a wireless router or a Starbucks to get online. However, for an additional $130 per size, the iPad comes with built-in 3G and a micro SIM card slot for GSM carriers. Even better are the data plans: $15 for 250 MB a month, or $30 for unlimited data. Both plans are through AT&T.
Also, you'll notice in the above picture that the 3G iPads have a black strip on the top. We don't know if it's just aesthetic or if it serves a purpose, but it does provide you with a quick-glance visual differentiation.