I second the motion for Seven Seas!-- its the best authentic Mexican restaurant i've found in the Grand Rapids area--and i've tried most of them. Kind of a hole-in-the-wall, but the food rocks!
The new Mexican place in Eastown is going in on Carlton, next to the Domino's pizza at Carlton and Lake Drive. They just put a sign out front last week (the name escapes me) and are currently working on interior remodeling. Not sure when they plan to open. I think they may have a tough time with visibility and parking, but i'm certainly wishing them the best-- my wife and I don't have much in the way of culinary skills and are looking forward to another restaurant within walking distance!