I've had several people ask me about this one especially now that in 3.0/MOSS there is the top nav bar and breadcrumb trails. I'll have to give props to Todd Bleeker for the steps on this one.

The process uses a Content Editor Web Part to inject some CSS and alter the page and hide the Quick Launch bar. You won't need to know CSS though, the snippet is provided below.

1. Go to your site you want to hide the Quick Launch
2. Click Site Actions > Edit Page
3. Click the Add a Web Part in any section
4. Put a "Content Editor Web Part" on the page
5. Click "open the tool pane" on the new web part.
6. Click "Source Editor" and paste the below in the editor window:

7. Click Save.
8. In the tool pane expand Appearance and change the title to read "Hide Quick Launch"
9. Still in the tool pane under Appearance select NONE for the chrome type.
10. Click OK to get out of the tool pane and exit Edit Mode.