Hi,
Look closely at your MagSafe connector. Are all the pins extended? Take a small implement and gently push on each pin. Does it extend again when you remove the pressure on it?
On my MagSafe, two of the pins, the outer two, have lost their springs, it seems. You can push them in and they just stay there. Thus often the MagSafe does not connect with the laptop, it does not charge, behaves as if it were on battery power ('cause it is) and the light on the MagSafe connector remains off.
I can get the pins out by holding its business end facing down and tapping on it (there's clearly very little friction on these pins) or by swinging the connector in an arc and then carefully connect it to the laptop. With a little luck / persistence and no big movements subsequent to connecting it, I can maintain the power connection.
Now, I'm entirely unwilling to send the whole thing (laptop and all) back for replacement, so I'm going to see if (or when) a replacement power brick is available. Fortunately, I work just a few blocks from an Apple store and just a mile or so from a Fry's. Call me foolish, but I'll buy a new power brick before I'll surrender my MacBook to repair an entirely separate component.
Randall Schulz
iMac 20" Core Duo; MacBook Pro Mac OS X (10.4.5)