I'd argue that being able to think laterally across multiple parts of the web browser is much more important for front-end development than having the typical CS background (algos, data structures, etc).
But the CS background certainly doesn't hurt. Especially when it comes to stepping back and figuring out how to do the same stuff with less code (i.e. making good architectural decisions and finding the right frameworks for the job), instead of diving straight in and producing a giant mess of code, which is quite easy to do on the front-end if you're not careful. (Especially if you throw JQuery in the mix -- at that point you have a hearty soup of DOM manipulation code.)
But the CS background certainly doesn't hurt. Especially when it comes to stepping back and figuring out how to do the same stuff with less code (i.e. making good architectural decisions and finding the right frameworks for the job), instead of diving straight in and producing a giant mess of code, which is quite easy to do on the front-end if you're not careful. (Especially if you throw JQuery in the mix -- at that point you have a hearty soup of DOM manipulation code.)