Login/sign-up in modal dialogs

I've been working on a web-platform for some time now.

As I was writing the usual "Sign-up" and "Login" pages I realized that a whole page was an overkill for such simple tasks so I decided to put them in modal dialogs.

Aside from depending on JS, what usability / user experience drawbacks could this bring to my users?

(If you don't know what a modal dialog is see here)