Open source saves cash, no mistake about it. That is, at least upfront.
The reason I say this is that I use Squid+Dansguardian to filter internet content at my school. Free, so it saves cash. However, everything else is Windows based. Why? Because we know it, and also because we have a ton of brilliant educational software packages that only work with Windows and Apple, designed by Shertson Software. I don't know if they would run under WINE, but even if they did, it would be too much of a hassle to try and get students to use it properly. Same goes for Dreamweaver and Studio MX.
Also, when it comes to networking, I love the power of Active Directory, and being able to lock the computers down fairly well. I don't quite know if there's something similar on Linux.
In the end, we each use what's best for us. I've been a few of the Tux labs facilities, and I think it's a grest concept. It gives computers to schools who never would have had the chance to own them. Unfortunately it's also rather basic, and once you get past a certain point, you have to be more open minded and use what is available.