A truly sustainable building will have a design which addresses a number of issues, including heating and cooling, water usage, environmental quality, and energy usage. Architects can deal with environmental aspects of building construction in a variety of ways, all of which are designed to increase efficiency without being cumbersome or detracting from the function of the building.
Some people love the idea of "greener" buildings, other people view them as too expensive with not enough results.
This article gives good information on what standards architects have to meet when designing a sustainable home or building.
The US Green Building Council has a lot of great information about different projects and ideas around the nation