That will really stick it to them--buy their product, pay them cash every month for service, provide advertising by letting people see you use an iPhone, but screw them over by not buying apps.
Check out this bit from Slashdot yesterday:
"The 4-month-old Cydia store is yielding notably higher sales for a few application developers than Apple's AppStore, and is reportedly running on over 4 million Apple iPhone devices, with perhaps 350,000 connected at any one time. In this store, developers are distributing applications they've written that push the limits of Apple's normal AppStore policies, with software to add file downloads to Safari, trick applications into thinking they're on Wi-Fi (for VoIP), and enhance other types functionality. You'll also find the popular Google Voice application, which was recently rejected by Apple."--http://bit.ly/199gAh