In my experience, most the carriers even out to roughly the same in terms of plans, with some minor variations. The only real deals I've seen come from retention plans. If you're with a carrier that carries the iPhone now, it may be worth it to call them, say you want to cancel unless you get a plan with everything you want for...say, $50 (depending on a lot of factors, just pulled that number out of the air). That will probably be how you get the best deal.
FWIW, I'm with Fido and pay around 80/month for unlimited evenings and weekends at 5, voicemail, called ID, 2500 texts and 6GB of data. That's taxes and service fees all in.
I agree, if you are happy with your carriers service and have no reason to leave other than cost and are not in a contract, then I would first let them know you are cancelling your service. You will be put to the retentions department, don't be afraid to haggle here. Be firm but polite. When my contract is up I will be doing the same and one points will be to be the best then can give me without a contract. No more contracts for me.
Surprised this thread isn't longer! I'm in the same boat: my contract is up soon and I'm looking around for the best iPhone plan. I'm currently with Telus and have negotiated myself a sweet deal over the years by telling Retentions what the other guys are offering. They will stall a bit and claim that they can't match things, but if you keep on the pressure (politely), you'd be amazed what they manage to do eventually.
Also, check with your employer if you are eligible for a corporate deal. They are often quite good. Even if you don't get it, you can use the plan as leverage in negotiating. That's how I got my current plan -- I told Telus that Bell was offering teachers a certain plan and the agent did her shtick about not being able to match that kind of thing, but lo and behold she did.
I've also found that going to those small booths in a mall are a better deal. The salespeople are usually young and eager to make a deal -- they'll throw in a few things if it means making a sale.
Look at the retailers which may have special rate plans, I got two iPhone through Times Telecom (Bell Mobility retailer) in a special share plan, $55 a month per line 500 combined minutes (additional lines would've added an extra 250 minutes). 250 additional incoming minutes per line, e+w after 6pm, CID, txt msging, and 6GB data.
2009 iMac 20", iPhone 4S for the wife and I, too many iPads
Are you backed up??? If so, why? Psyllium Fibre is not free but will do wonders!