I've been burned several times by missing semicolons when minifying my code. Now I make sure they're always there. I'm also running everything through JSLint.
I think JSLint can be a big help, especially with semicolons so I made a quick jQuery plugin to run things through it on page load. So I know exactly when something may be wrong.
I'm using the Javascript Tools bundle for TextMate. It runs JSLint every time I save a javascript file.
However, I've never had a problem in the cases where I have indeed minified unsemicoloned code. Has anyone else, or is the problem just theoretical? If others have encountered an actual problem, I might have to rethink this strategy for larger codebases.