If you want to use your iPhone, the only option is the Wahoo Fitness bluetooth transmitter (speed and cadence).
While your charged iPhone will last for your short rides, a computer will be easier to mount and occupy less space (note that you'd need to buy a mount for your iPhone). You also might also have to worry about protecting the iPhone from weather and being dropped (or stolen) while mounted on the bike.
Personally, I like using a computer so I don't have to worry about having enough charge on my phone (I'm doing much longer rides).
I suspect that, "soon", there will be wireless bicycle computers that will used Blue Tooth (and whose transmitters will also be useable by your iPhone).
A fair number of people use the iPhone approach but it isn't going to be cheaper than a computer. If you want to also track your route, the iPhone approach will (likely) be cheaper.
You can get the Cateye Strada Double Wireless (speed and cadence) for about $60.
None of these (especially the wireless versions) are hard to install.
Installing whatever you choose might be a good way for you to get familiar enough to deal with alignment issues in the future.
Last edited by njkayaker; 06-04-13 at 07:33 PM.