http://social.msdn.microsoft.com/Forums/sqlserver/en-US/5bff42a4-966c-424f-8e51-67b6e17814d3/has-the-intellisense-behavior-changed-in-ssms-2012?forum=sqltools

Basically says "go to Edit->IntelliSence-> Toggle Completion Mode " however the setting is not permanent, CTRL+ALT+SPACE is the shortcut to change it.

You can also just hit tab instead.