This has been indirectly stated a couple times in this thread, but I wanted to expressly point it out on the off chance that anyone isn't aware of it. While it appeared to be in the beginning, the $10 extra fee is not only for 4G capable devices. So don't think just because the newly christened HTC Arrive doesn't have WiMax means you'll be able to avoid this charge. The fact is, any new activations of this or any other smartphone starting January 30th WILL be charged the fee.
As I mentioned before, this isn't breaking news in the slightest, but just in case here's a link to one (of surely) many blog posts detailing the change.
Sprint notices you're using data, is adding $10/month fee to all smartphones
P.S.
On a more directly thread-related note, I am also inexpressibly disappointed at the lack of WiMax on the Arrive. It's a deal breaker for me as although my profile lists my location as Duluth, I spend a great deal of time in the Minneapolis area and would love to be able to utilize the 4G there.