OK, so now I'm confused. So you are going to buy a new MacBook Pro, I think you are asking if you can use a 21.5" iMac as an external display for the MBP. If so that functionality is called Target Display Mode. The answer depends on what year iMac you have. If it's a mid 2011 or you plan on getting a 2012 21.5" iMac then yes as they both have Thunderbolt connections. However if your iMac is 2009 or 2010 iMac then no as they do not have Thunderbolt connections, TDM will not work with a new MBP that has Thunderbolt.
Please carefully read:
Thanks for reading my mind.
I have a later 21.5/purchased in 10 of 2010. eek!
I'm trying to figure the best way to go.
I currently work on magazines and will be adding more.
I need the laptop to travel with...so that means that I thinking
i'll get the 15" Mac Book Pro and either get a 27" monitor or the 27" iMac. I like the real estate that it offers on the 27". Will get rid of the 21.5 iMac (of which the HD just bit the dust). Not good for PR, but it did.
Thanks for your help.