If you have an iPhone, you can pair it with your iPad, using the iPhone as a wireless router.
There is software out there - I use MyWi - on a jailbroken 3Gs.
Apple charges an extra $160 or so for the 3G enabled iPad. MyWi costs $10.
No extra sim card needed, save $140. Short range, password enabled etc., but a little thirsty on power. Slip on a battery pack ($20) and you're still $120 ahead without the hassles of an extra contract etc.
FWIW, the newer generation Android OS phones do the same trick out of the box, no software purchase needed.