Semprons are the current series of 32-bit athlons. They gave the Athlon name to the 64-bit series chips.
I can see it now:
"Hello, XXX Tech Support, how may we help you?"
"Yeah, I just bought this Athlon, and it won't run Longhorn."
"Can you tell me what model you have?"
"Um, sure, lemme check here... it's an Athlon XP 2800."
"I'm sorry sir, that's a
Sempron, you'll need an
Athlon to run Longhorn."
"Wait, the box says it's an Athlon, and Longhorn says it'll run on Athlons."
"I'm sorry, sir, that's NOT an Athlon."
"But the box..."
"Any further questions today, sir?"
"No thanks, I think I'll use one of the BSDs instead."
"You know the SCO Group is prosecuting Linux licensees, right?"
*CLICK*
EDIT: THIS is PRECISELY why I buy Intel processors. If Microsoft starts locking them into Longhorn,
then I'll use AMD, but dealing with Stupid Bastards Inc Tech Support is NOT something I look forward to doing.