http://www.istartedsomething.com/20090725/changes-to-the-windows-7-install-process/:

Up and until RTM, the ISO image files of Windows 7 that Microsoft distributed (and third-parties distributed) are labeled with a version. For the most part, this has been “Ultimate”. Even though the install image “install.wim” in Windows 7 still contains four versions, one version is automatically installed determined by a file called “ei.cfg” in the “sources” directory on the install disk.

To get around this limitation, simply deleting the “ei.cfg” file will allow you to choose a version to install (illustrated above).