How to convince parents to get me a smartphone?

OK so my mom said no to even the cheapest smart phone for AT&T, the HTC Aria or whatever is like .01 dollars with the upgrade (october). BUT they can dictate because they both have an iPhone 4 with the most expensive data plan (25) dollars. Ill be fine with the lowest data plan (15) dollars a month. they claim i dont need one because i have an ipod touch??? IM NOT GONNA BE STUCK WITH ANOTHER CRAP PHONE AGAIN. please help! btw im 15. and RESPONSIBLE.