Another alternative is to use a hardware load-balancing device. In the past, people have been afraid of these, thinking that they're nothing more than IP-based network load-balancers. Well, times have changed! Windows Server 2003 exposes all of the performance counters and the session directory to external devices (i.e. these load-balancers). Now, you can have a hardware load-balancer that can intelligently route users to a server based on processor utilization or the overall number of sessions. Commonly referred to as "Layer 4-7" routers (since that's the part of the OSI stack they operate in), you buy them based on the number of servers you have, rather than the number of users you have. A $2000 load-balancer might seem expensive until you realize that it can support 8 servers, each with 100 or so users. $2000 for 800 users is not bad. (Of course these load-balancers can approach $20,000-$60,000, so they're not always the best solution.)
So, who makes these load-balancers? Several companies do--some that you've heard of and some that you haven't. The post popular ones for Terminal Server environments are F5's BIG-IP line of products. Nortel offers hardware load-balancing via their Alteon products, and Foundry's ServerIron products do the same. Finally Cisco's 11500 series of routers also offer Layer 4-7 content switching.
All in all, there may be a void in the market for a simple, easy, and cheap software-only load-balancing solution. Until then, these hardware devices do everything that's needed and more.