If you have an iPhone, you might be OK with wifi
For myself, having an iPhone that gives me internet/etc via 3G, it did not make sense to pay the extra $$$ (up front + recurring data plan charges) to get 3G on the iPad.
I basically use my iPhone as (1) a mobile phone and (2) a mini/pocket iPad. My internet use on the iPhone, for the most part, involves navigational help/Google Maps, quick Twitter/FB/FourSquare updates, short emails, and quick "who wrote that song" type of Wikipedia searches.
Longer internet sessions (reading the news, checking RSS feeds, catching up on Twitter/FB) are things I do at home, where I have wifi. I do travel on business, but now that many/most decent hotels have wifi, I can usually jump on wifi even while travelling just as I do at home—in the evenings, at the end of the day.
If you don't have an internet-capable mobile phone, then maybe the iPad 3G makes more sense.