Easy enough question (at least it was for me).
Think of all the places you visit, can you access wifi? I plan on mainly having this at home and at work, both have wifi. Maybe various family member's homes, but they have wifi. The only place I could think of where I'd use 3g is when I go to a convention and there's either no wifi or they want to charge for it. I go to maybe 3 conferences a year, and one of them was in january. So $130 +$15 or whatever each month I go doesn't make too much sense to me. Plus I can look into jailbreaking my iphone again if needbe.
Now, if you are frequently in areas where you'd like to use the ipad, require internet access, and no wifi available, then 3g may be worth it.