I wear earplugs most nights and I still hear the alarm in the morning. As others have said, I think it's because 'plugs block out the low frequencies better, so the 'alarm gets through.'

However, what's crucial for ear plugs to be effective is correct insertion. For example, when I'm on a flight I see a lot of people just "cramming" the foam plugs in their ears. That's next to useless - You might as well not bother.

You need to roll them between your thumb and forefingers for 10-15 seconds into a long skinny compressed cylinder, then reach over your head, pull on the ear to straighten the ear canal, then slide them deep down into the ear canal as far as you can and wait about sixty seconds for the plugs to 'expand'. Makes a HUGE difference.

Slate.com recently reviewed ear plugs: http://slate.com/id/2118800

Their favourite is also my fave: http://earplugstore.stores.yahoo.net/holemaxuffop.html

There's a thread on ear plugs here:

http://www.flyertalk.com/forum/showthread.php?t=459511