Like I said you can install the iPad baseband. How important is GPS to you? Do you want to use your iPhone as an iPod Touch or as an iPhone? If you really dont want to install the iPad baseband, but want to make calls I suggest you either buy a factory unlocked iPhone, buy an iPhone that can be traditionally(no baseband flash) unlocked, or download a VOIP calling app.
I've read rumors that a regular unlock has been found for 05.14bb and 05.15bb but wouldn't be released until the iPhone 4 unlock is released, but keep in mind this is a rumor and holds no definite validity.