Promote the plus points:
Lower TCO (smaller energy req, easier maintenance, higher reliability, 10 yr lifespan of clients)
Shared processing power (upgrade server and everyone benefits)
Just remember that you they aren't for everything: can't really use them for multimedia tasks and you may as well turn off sound as it's terrible - latency causes sound to reach you seconds after video if at all.
Shame, since the linux implimentation is much better in that regard, feels much more like a full box with LTSP.
Normal application use including flash is fine (full lab of flash games does slow things down a little ) Open Office, educational apps all good.
My server has 2 quad core xeons and 8GB of ram - don't scrimp on the ram.
Links:
Windows 2003 Terminal Services (Part 1)
Microsoft Windows Server 2003 Terminal Services
EDIT: forgot to mention if it's a new deployment, you might consider server 2008. It has a few improvements to ts such as application deployment as opposed to desktop deployment - just the application window is transferred to the thin client.
However atm for that I believe you need a windows based client - at least until the open source guys reverse engineer the new bits. Worth keeping in mind if you have linux clients.