So I posted a week or so ago about DRY themes for WordPress. Turns out, this has been done already (I thought it might have been), and in an even more awesome way that avoids my conditional-filled logic statements.
Check out scribu’s Theme Wrappers, the most brilliant handling of this issue yet.
With a little bit of extra code in your functions.php file (or in a plugin–after a little more testing I’ll probably incorporate this into the rach5-helper) you have a perfect setup that lets you define a base php file (wrapper.php in his example, but I prefer base.php) and use WordPress standard template names to fill in the gaps, saving you time and energy and keeping you from needing to maintain a 20 line conditional check.
This is an awesome system and absolutely worth checking out if easily maintainable themes is something that matters to you!