Patrick, TELCEL told me there is no problem. Take your iPhone and passport to a TELCEL Servicios de Cliente either in Ensenada, Cabo San Lucas or wherever you will be most of your time as the number you receive will be for that area code and every call you make outside of that code will be priced as a long distance call plus the per minute cost.
The cost of a chip is now MXN 150.00 but includes some air time. The cost per minute for a new client is MXN 4.50 per minute. Note that in Mexico the charge is per minute so a call that is one minute one second will be charged as two minutes. Text messages cost about MXN 1.15 per text no matter if it is national or international.
When you go to the service
centre I suggest you take a Spanish speaker with you if you don't speak it yourself as they are not very used to dealing with foreigners.
When buying pre-paid cards I suggest you buy the MXN 500.00 card as this will give you 900 pesos of air time cost. Lower cost cards have bonus minutes also but on a reducing sliding scale. Always buy your card at a reputable sales point as street vendors may sell you a card that is fake. Then try and find them again.
On prepaid card I am not sure how 3G works nor what it costs but if you have wifi
available that is always superior.
There are other companies but TELCEL has the best coverage and the most reliable service
Hope this helps and have a good time on the Baja