Stress testing a Citrixfarm, a Terminal Server environment was never easy. You had to use selfmade scripts , edit the development kits made available by Citrix and Microsoft and other toolkits. A dutch company called Denamik created a freeware utility for 150 users called LoadGen

A summary ripped from there site what Loadgen is capable of

• Non-intrusive session launcher for individual Citrix, Microsoft Terminal Server (RDP) en Provision Networks  servers or large farms.
• Launching and managing thousands of sessions was never so easy.
• Fast and easy deployment of LoadBots on multiple workstations or servers.
• Create hundreds of test-users and multiple test-profiles in minutes.
• Get DeNamiK LoadGen up and running within one hour.
• Test the whole production farm during off hours with minimal preparation.
• Use your favorite scripting method. Compatible with Wintask, Winbatch, Auto-it, script-it,
• VBscript and all the other application test scripts.
• Full session loadbalancing on all LoadBots, this prevents overloaded workstations launching ICA or RDP sessions.
• Evaluate the Freeware version of DeNamiK Loadgen for an unlimited time, unlimited LoadBots, unlimited server and a maximum load of 150 concurrent sessions.
• Install the DeNamiK LoadGen components on Windows 2000 (Service Pack 3+), Windows XP and Windows 2003.
• Microsoft .Net Framework 1.1, 2.0 or 3.0 necessary.

Brain Madden also wrote an article about this tool and you can read it here

Normally you would have to script much your self but I already read that Denamik is automating lot's of stuf also in the future. So keep an eye on these guys and test the product and see if it's worth buying it. It could save you much time stresstesting an environment.