As business and commerce go mobile, payment technology is playing catch-up, trying to come up with a simple and powerful way to allow businesses to accept credit card payments on the go (because who even uses cash anymore?).

One possible answer is PAYware, the mobile-payment system coming from point-of-sale giant VeriFone. PAYware finally saw its app go live in the iPhone App Store today, bringing its secure credit card transactions to the iPhone, and making itself the first real competitor to Twitter co-founder Jack Dorsey’s, Square.

PAYware, which looks like half an iPhone case, is a credit card reader that collects, transmits, and confirms payments for you anywhere you get iPhone service. Customers can sign for their purchases right on the screen with an included stylus, and with one tap have a receipt sent to their email address. On the merchant side, everything gets synced and tracked with VeriFone’s Web interface. It’s simple, and totally mobile — business can happen in-store or across the world.

So far, so Square-like. But the niche VeriFone is trying to carve out with PAYware is for businesses that need more security and more support than, say, vendors at a flea market. VeriFone is an established player in the point-of-sale-system world with a heavy emphasis on security, and is implementing similar standards with PAYware. Users are required to have a “merchant account” (an agreement with a bank that proves you’re a legitimate business), and there’s some hefty encryption from VeriFone in every part of the transaction.

That makes PAYware a secure, trustworthy system — with a lot fewer potential users than Square. First of all, it’s only on the iPhone 3G and 3GS, while Square is quickly on the way to working on everything with an Internet connection and a headphone jack, and for everyone from Fortune 500 companies to hot dog vendors. PAYware has said it will support other devices in the future but has both hardware and software hurdles to clear that Square — which just plugs into a headphone jack — doesn’t have to overcome.

Users also pay a premium for the extra layer of security from PAYware — there’s a $49 activation fee, a monthly fee, and a fee for every transaction. Square, by comparison, only charges the transaction fee.

For businesses concerned with security and wanting to work with an established player like VeriFone, PAYware may have some staying power. But as far as adoption by average Joes, something Square’s certainly aiming for, PAYware’s barriers are a littler higher. Whichever model wins out, it’s going to bring a whole new accountability to “dude, I’ll totally pay you back.”