my company was recently looking for a terminal service solution. Because we sell our software to mostly small companies, it is sometimes too expensive for them to buy MS Server 2003 just to have 5-6 simultanous rdp connections, so we were looking for alternative and we found http://www.xpunlimited.com/where they state
that their product
'XP Unlimited turns your Windows XP Professional System into a full blown Terminal Server, without any limit.'

we found them just last week so haven't deployed their software yet, but probably we'll go out and acquire.
Price can't be beat; unlimited connection terminal server on plain old winxp for just 125$

from my undertanding, they found a way to expand already present RDP in winxp pro.