Q: video input to Imac 27"

If both you laptop and imac are thunderbolt versions then yes.

Using a 27-inch iMac (Late 2009 or Mid 2010) as an external display

The previous poster is assuming your iMac is a 2011 machine. If it's a 2009 or 2010 model then please refer to:

http://support.apple.com/kb/ht3924

The feature you are interested in is called 'Target Display Mode'. As X423424X implied the feature is supported on the 27-inch iMac's with built in Thunderbolt support, but it was also supported on the older 27-inch iMac's wihich had the Mini DisplayPort. This article will provide more information:

http://docs.info.apple.com/article.html?path=Thunderbolt/10.6/en/30822.html