Steve Smith, the MVP extraordinaire from the UK and part owner of Combined Knowledge along with Penny Coventry, MVP extraordinaire from PPP Consulting just released a white paper showing how to configure a load test end to end using VSTS 2008. This is great because long ago I promised to do this on my blog but never got around to it so Steve and Penny took a burden off my shoulders. ;-)
I haven't completely reviewed the paper, but the pieces Steve showed me yesterday were great. I'll add that you should take a look at the WSS and MOSS load test examples posted on codeplex as part of the SPDataPop tool project. These examples will save you a lot of time and they are built to leverage CSV files for variables such as web application urls, team site urls, user credentials, and search terms. These load tests are very similar to the ones the SharePoint PG test team uses for stress testing of new releases.
So why care about stress testing your SharePoint environment in the first place? Microsoft will probably never be able to provide concrete capacity guidance for your unique SharePoint situation. There are far too many variables to control. You should stress test every environment you manage before deployment to uncover capacity and configuration issues before they bite you. Neither would it be a bad idea to periodically test each environment after deployment since capacity issues can develop over time due to poor maintenance practices and worsening environment conditions. (network, AD, storage, etc)