I deliberated a lot whether to go wifi+3G or wifi only. Here in the UK, it was an extra £80 for the 3G model (16gb) so it wasn't a lot of money. I decided on the wifi only version and here are my reasons:
I'm on an unlimited data contract on my iPhone, so I thought tethering was a good option. This actually works well.
I have BT Broadband in my home, which allows me access to BT Openzone and BT FON hotspots.
3G is very hit and miss in Birmingham- most of the time, my phone is running on Edge. I go to London a lot and you're lucky to get a phone signal there, let alone 3G or Edge.
I wasn't keen on the black strip on the 3G model- I like the all silver look of the wifi version. Purely aesthetic I know, but that's me!
If I go abroad, 3G is an astronomical cost, so getting the 3G iPad wouldn't have been worth it. I went to Mexico last year and had to switch off my iPhones' 3G capability. There are lots of horror stories here in the UK about customers who've been on holiday abroad and got a huge bill when they've returned, without knowingly using any data. I was charged £10 when I first went to Mexico, for data being consumed in the background over just 3 days. I was charged £2 for receiving a voicemail that I never listened to!
Last but not least- I don't plan on taking my iPad with me when I'm out. I can't take it to work, and I really don't want to be carrying it around when I'm out socially. Again, when we were in Mexico, i noticed the US crowd brought Ipads, laptops, macs etc to dinner with them whereas the brits didnt. Im guessing it's probably more socially acceptable to do this in the USA, but take an iPad into the pub here, and everyone will laugh at you (or steal it)..
I know consumers in the States have better data deals than we do, so I can see their point of view also and I can see 3G being more attractive there.
In hindsight, the only thing I'd buy 3G for is the GPS for Find My iPad. I say this, as I wasn't aware there was no GPS chipset in the wifi only version until I read this thread!