A month before Tuesday’s Apple iPhone event, before everyone was 100% sure that Sprint would be acquiring the new iPhone, rumors surfaced that Sprint would be launching the new iPhone with an unlimited data option.

Just two weeks ago, speaking at GigaOm’s mobilize conference, Sprint’s CTO Stephen Bye said that Sprint was committed to unlimited data.

Now we get confirmation that Sprint will indeed offer unlimited data plans for the iPhone 4S.

Sprint spokeswoman Michelle Leff Mermelstein told All Things D that the carrier will offer unlimited data plans starting at $69.99. This plan will include unrestricted data as well as 450 voice minutes and unlimited mobile-to-mobile. If you want unlimited everything, data and voice, you’ll need to shell out $99.99 a month.

On top of those base prices, you can also expect to add a $10 smartphone surcharge.

This is undoubtedly a big draw for Sprint. They are now offering something that both AT&T and Verizon don’t offer anymore – unlimited data on an iPhone. New customers signing up with AT&T or Verizon will learn that their data plans now work on a tiered pricing structure.

Will Sprint be able to keep unlimited data plans? Back in September, analysts speculated that Sprint might not be able to offer unlimited indefinitely – that they might opt to switch to a tiered structure once they have pulled a number of users away from AT&T and Verizon.

As far as all the new traffic that will come from gaining the iPhone – Sprint says they can handle it: “We have every confidence in the ability of our 3G network to handle the influx of devices we expect to get,” said Mermelstein.

Sprint CTO Stephen Bye, last month at the mobilize conference, said that there was “clearly a cost to support unlimited,” but they were committed.

The iPhone 4S will drop on October 14th on the Sprint network.