I know that I am on the new billing system as well, so that seems to be somewhat of a common denominator at least. I know it caused issues when I tried to port in the old number.
I am in the same boat as you as it pertains to the OS issue. I clearly in every instance have been far ahead of any of the people I have talked to at Sprint when it came to the computer or the BB. I actually asked the "technical" person the last time if she "actually knew anything about any of the devices I was asking her about". I kept asking specific questions only to hear her frantically typing in the background. She was at least honest and said no, she just reads from the data base. She sends me to the BB expert. Same question, different answer but the truth was clear. Made the same point about the OS, it went nowhere.
Luckily for me this is not critical, but it is annoying. I don't actually care if it ever tethers in windows, but it would be nice to get it to hook up to the Mac. If I ever actually hear from someone on my Windows issues I will let everyone know. Mine will not work under Windows with the latest BB software and the latest Sprint software either.