Neowin reports the first Windows Phone 7 update is bricking some phones. The site offers some advice on how you may be able to fix your device if this issue affects you:
Unplug USB form computer. Charge phone or put in charger.
Turn off phone.
Turn on phone using 3 buttons - power, camera and volume up/down rocker. Keep all 3 held for 10-15 seconds. Do not release when it asks if you want to format the SD card.
Keep holding until it says "download mode". This will reinstall the OS.
If phone does not reboot in 15 mins just power it off and back on.
Re-setup phone, update firmware through Zune software and then re-sync your documents, music, pictures, videos, etc..
WPCentral reports Microsoft is expected to push a new Windows Phone update in 3 days to address the bricking issue, and notes it mainly seems to affect users of Samsung phones:
Reportedly it's a firmware issue with Samsung, more accurately those with firmware versions JI9 and JJ4 are experiencing problems, while the JK1 firmware, according to ZDnet, are doing okay. While we have no way to confirm that 100%, it seems best for those users to avoid the update. In fact, that may be easier than it looks since from our perspective (here in the States) Microsoft has either yet to or has indeed put a halt to rolling out the update (both my Quantum and HD7 have yet to be flagged for an update, even when force-checked in Zune desktop. Edit: Carriers may be blocking this update).