To launch Report Builder, use one of the following URLs (also documented on MSDN):

http://<servername>/reportserver/reportbuilder/reportbuilder.application (Full Trust)
http://<servername>/reportserver/reportbuilder/reportbuilderlocalintranet.application (LocalIntranet permissions only)

In addition to this, several launch parameters are supported:

To automatically open a specific report:
ppend "?<reportpath>" to the URL (e.g. http://.../reportbuilder.application?/My+Reports/Quarterly+Sales+By+Region)

To automatically load a specific report model:
Append "?model=<modelpath>" to the URL (e.g. http://.../reportbuilder.application?model=/Models/Adventure+Works)

To automatically load a perspective of a specific report model:
Append "?model=<modelpath>&perspective=<perspectiveID>" to the URL. Note this is the ID of the perspective, not the Name.