In the Beta 2 release of Team System you can only launch Team Builds from the Visual Studio IDE. The SDK has some sample code that uses the API, which I haven't tested, if you have and it works, your sample will be appreciated on this site. The solution I implemented was the workaround from the link below, the only problem with that code is that the values it sets for the properties are not correct in Beta 2 (maybe it was released prior to Beta 2), I made it work by assigning the name of the Build Type I want to run to the ConfigName property, instead of the portfolioProject string, that is the only change needed for this code to run: 

http://lab.msdn.microsoft.com/productfeedback/ViewWorkaround.aspx?FeedbackID=FDBK25583

EDIT (7/22/05): See my subsequent blog entry also: http://weblogs.asp.net/omarv/archive/2005/07/18/419763.aspx