When I get my virtual mobile bill each month, I’m reminded how much more it costs me to own a smartphone versus my old flip-phone standby. While I’ll likely never go back to paying $34.99 a month (those were the days), the high monthly cost of owning a smartphone is — slowly — coming down, with a catch. More users will be required to have data plans, with the latest update in monthly fee requirements, which means more reach for your non-SMS mobile marketing and advertising campaigns.

Verizon Wireless and AT&T Mobility, the two largest operators by subscribers in the US mobile market, announced price cuts of up to 30% to remain competitive with lesser used, low-cost cellcos like MetroPCS and TracFone, starting today. Both companies are reducing their $100 per month unlimited voice plans with $70 per month alternatives.

Digging deeper, there may not be such a huge difference in cost for AT&T and Verizon customers. Verizon recently increased charges for heavy data users and now requires all 3G phone users to sign up for a $10 per month data plan.

Reduction in monthly fees for smartphone plans will continue to spark smartphone adoption. Verizon requiring 3G phone users to sign up for data plans effectively pushes users to buy a more feature-filled smartphone, for a slightly higher monthly fee. Over at AT&T, iPhone customers landed a good deal, with the new $100/month plan for unlimited calls and data. As monthly fees creep down, smartphone adoption will continue its climb.