Literally the only way is to sync them with the same Zune client and restore the new one from the old backup. I found it easier just to download them all again. It's not like there are a ton of apps for WP7 (that you'd want) and the one's you purchased will automatically be recognized as long as you're signed in with the same live account.
Exactly, except I'm anal. So I keep a running list of current apps/deleted apps in OneNote (synced to my phone) w/app name, type (App/Game) developer and price. The ability to print out the list works well with 170+ apps, plus another 80 deleted. Makes reloading much easier. Adding apps only takes a few seconds and dragging removed apps to the deleted list is a breeze.
On some reinstalls I have lost apps, so tracking down by developer is helpful.
You could always go to Windowsphone.com, log in with your live account, hover over your name, click 'Account' and you'll see a list of apps you've previously installed. You can click the links right there to install directly from the web.