T-Mobile USA has always kind of taken a "don't ask, don't tell" approach to tethering by never offering an option to purchase tethering, but by offering devices that supported it through apps or utilities. According to Boy Genius Report that is going to change on November 3rd when T-Mobile will add a tethering option for selected smartphones for $14.99 per month. This will just give you the official authorization to what you may already have been doing on your Google Nexus One, T-Mobile Touch Pro 2, Nokia N8, Nokia N900, or other smartphone in the past. It's not clear if they have the ability to turn this off on you if you are already using it, but there is a statement in the BGR document that those of you already doing it will receive a message you need to enable this service.
The T-Mobile Tethering/WiFi add-on service will not increase your data consumption allocation so you will still get unlimited data with a 5GB throttling limit. The price is much lower than all of the other major US carriers and honestly the $15 amount seems to be right in line with what I stated was reasonable in the past. However, I still think if you are paying for 5GB, 2GB, or whatever your limit is, why does it matter how you use it and why is there any charge at all? If you want to truly limit consumption, just have tiered pricing and let people get to that limit however they desire.
So now that T-Mobile is joining the ranks of sticking it to you for tethering, let's take a comparison look at all four major US carriers to see how tethering from a smartphone stacks up.
A good point was made by my Twitter followers that the Palm Pre Plus and Pixi Plus on Verizon get FREE WiFi hotspot tethering included with their data plan. This is how it should be for ALL smartphones, IMHO.
So as you can see even though T-Mobile is going to start charging customers, they still have the lowest cost with unlimited data so are clearly the best deal with the fastest speeds. Sprint is great for those with heavy data needs while AT&T and Verizon are just fine for those not consuming more than 2GB of data.
4G (WiMAX) and HSPA+ speeds are currently showing about the same download speed in testing by people around the Internet. I have been seeing almost double the HSPA+ speeds with my Sprint EVO 4G in a 4G/HSPA+ area though so your download speeds may vary.