If you haven't gotten your iPad yet, I recommend you get WiFi + 3G version. Whether you own an iPhone or Android smartphone (with tethering capabilities), it might be troublesome in the long run as mentioned above by ysc.

Plus, if you're already on a data plan for your smartphone, I'm sure your operator would have a special package for additional data plan for the tablets.

FYI, both my iPhone + iPad are on Maxis data plans. iPhone is on monthly RM90 (500MB) & iPad is monthly RM80 (6GB).