I use [Soocial](http://www.soocial.com) to keep my contacts synced between Google accounts.  I use Google apps for my email, but also use a Google Voice account.  This is the easiest way I've found so far to keep my contacts in sync. 

Addendum: [Soocial.com is shutting down July 31, 2013](http://blog.soocial.com/2013/07/01/we-are-shutting-down-soocial-com/)