If you are interested in buying an iPhone 3GS handset, Boutika.ca is selling the handset alone (no plan) for a good price right now, for only $506.62.
You can order it in white as well. Remember to use the exclusive Bargainmoose coupon code for an extra 12.5% discount off the marked price - bargainmoose125. It makes the iPhone $506.62 in total.
The phone is unlocked, so you should be able to use it with whatever sim card you are currently using.
Just to price compare, the iPhone 3GS is $549 on the Apple store - and that's only for the 8GB model. At Boutika, you're getting the 16GB model for even less. Maybe this is a good deal because the iPhone 4 has just come out, so the 3GS is slightly out of date.
I know a few people with iPhones, and they do nothing but rant and rave about how good they are. I do like the design and aesthetics of them, but I don't think I would personally fork out $500+ for one of these.