Simple's new $40/month Nationwide Talk Text and 3G Data Plan includes unlimited, voice, text (including international texting), picture messaging and 3G data.
That price undercuts the previous low price leader Straight Talk, whose Unlimited everything plan is $5 higher at $45/month. Simple has the added advantage of allowing you to bring your own device (BYOD) unlike Straight Talk which limits you to a small selection of mostly feature phones. The least expensive other BYOD unlimited everything plans that include 3G data are H2O's $60 plan and T-Mobile's $70 offering.
You can use any T-Mobile USA or unlocked GSM phone including iPhones, Android and Windows Phone devices on Simple Mobile. However you will only get EDGE (2.5G) data speeds with the iPhone as it doesn't support T-Mobile's 1700/2100 3G frequencies.
One potential downside to this plan is that you are limited to T-Mobile's native network with no roaming (coverage map). This is less coverage than T-Mobile's own prepaid plans which allow voice and text (but not data) roaming on many rural and regional carrier's networks. T-Mobile's native coverage is roughly comparable to Boost and Virgin Mobile's Sprint native coverage.
Also unlimited data is never really unlimited. There's always a cap. Simple isn't saying what it is but in the past they have capped their unlimited data plans at a relatively low 1GB. If you exceeded the cap you lost data for the rest of the month.
Simple offers a couple of upgrades to the basic $40 plan. $10 more gets you BlackBerry BIS service which gives BlackBerry users access to push email and BlackBerry Messenger. A $60/month plan provides 4G HSPA+ data to supported devices.
I expect Simple Mobile's new $40/month plan to spark a new round of unlimited prepaid price cuts.