When nosing around the mainmenu.res file in the dlc folder I noticed that there is a menu button called 'BtnPlayCommunityMaps' as follows:
codestuffs
// Community Maps
//"BtnPlayCommunityMaps"
//{
// "ControlName" "BaseModHybridButton"
// "fieldName" "BtnPlayCommunityMaps"
// "xpos" "88" [$GAMECONSOLE && ($GAMECONSOLEWIDE && !$ANAMORPHIC)]
// "xpos" "63" [$GAMECONSOLE && (!$GAMECONSOLEWIDE || $ANAMORPHIC)]
// "xpos" "88" [!$GAMECONSOLE && $WIN32WIDE]
// "xpos" "63" [!$GAMECONSOLE && !$WIN32WIDE]
// "ypos" "250" [$GAMECONSOLE]
// "ypos" "228" [!$GAMECONSOLE]
// "wide" "220"
// "tall" "20"
// "autoResize" "1"
// "pinCorner" "0"
// "visible" "1"
// "enabled" "1"
// "tabPosition" "0"
// "navUp" "BtnQuit" [$GAMECONSOLE]
// "navUp" "BtnEconUI" [!$GAMECONSOLE]
// "navDown" "BtnPlaySolo"
// "labelText" "Play Community Maps" //"#PORTAL2_MainMenu_Solo"
// "style" "MainMenuButton"
// "command" "CommunityPlay"
// "ActivationType" "1"
// "FocusDisabledBorderSize" "1"
//} We all know that the new editor coming will let anyone make maps for portal and I'm guessing that this would be the button that opens the GUI to load a map.

Would be disappointing if Hammer made maps couldn't be included.

What happens if you uncomment all that stuff?

Nothing unfortunately.