that effect will feel more like an "obstacle".
If at all, the modal dialog is the obstacle, not the effect.
I don't even agree it's an obstacle. For the average geek, choice is good. For the average user, guidance is more important.
The dark background gives a clear signal: I want your attention here. A big friendly cancel button is the only thing you need for your users that don't want to log in to be happy.
let me shoot down the two arguments I can construct against the dark background:
1. There are use cases: User A clicks "Login" and continues to do so. User B clicks "Login", but then realizes he'd rather click the "What's hot" link.
For user B, the dark background is good: he knows he needs to click Cancel before he can click "What's new". If you are modal, the background isn't interactive anyway.
User A would never notice the restriction. However, even for her, you lead her focus to the login controls (the "guidance" argument above).
2. Seeing what's behind. There are modal dialogs for which, to complete them, you need to see what's in the main window / below them. But that's bad usability: the application data the user requires to fill out a modal form should be available from the form. If you need to see e.g. a graph, a side-by-side arrangement is usually better.