First you need to loose calls with her with the phone you are useing now. Now do this for a couple of weeks, and keep complaining about this (at this point don't mention the treo). Price the most expensive treo and tell your wife...boy I need a new phone and this is the one I need but boy it's too expensive. Then start hunting on ebay and wait for a really good deal, I've had good luck with refurbished. Before you know it your the hero, you save hundreds of dollars and what a great connection you now have.
Now I'm not speaking from personal experience of course.
