They are scanners not apps
Actually they are Walkie Talkies or handheld transceivers. NOT SCANNERS
Seriously, if (BIG IF) these apps start becoming popular the TELCOS will simply make it difficult or impossible to use them.
The TELCOS make their money from the Voicecalls, SMS and DATA plans, anything that will impact on that income will be soon stomped on or CHARGED for.
Skype is probably the best App for this concept anyway. Cross platform support, Existing user base, and Major players supporting it.