I'm having the same problem. I had successfully connected and programmed the unit, and now No Joy.
Uno R3, windows Vista 64-bit, Arduino IDE 1.0.1
Tested different com ports, tested different USB cable, tested different USB ports, tested on Win 7 machine.
Power led is green, and digitial 13 led is yellow.
Com ports in agreement between windows device manager and Arduino IDE
Loop back test successfully echo's the input.
==========================
Verbose out from monitor:
Binary sketch size: 1,084 bytes (of a 32,256 byte maximum)
C:\Users\Patrick Eidemiller\Projects\V3\SupplySolutions\Arduino\arduino-1_0_1\hardware/tools/avr/bin/avrdude -CC:\Users\Patrick Eidemiller\Projects\V3\SupplySolutions\Arduino\arduino-1_0_1\hardware/tools/avr/etc/avrdude.conf -v -v -v -v -patmega328p -carduino -P\\.\COM6 -b115200 -D -Uflash:w:C:\Users\PATRIC~1\AppData\Local\Temp\build8167292190882310592.tmp\Blink.cpp.hex:i
avrdude: Version 5.11, compiled on Sep 2 2011 at 19:38:36
Copyright (c) 2000-2005 Brian Dean,
http://www.bdmicro.com/ Copyright (c) 2007-2009 Joerg Wunsch
System wide configuration file is "C:\Users\Patrick Eidemiller\Projects\V3\SupplySolutions\Arduino\arduino-1_0_1\hardware/tools/avr/etc/avrdude.conf"
Using Port : \\.\COM6
Using Programmer : arduino
Overriding Baud Rate : 115200
avrdude: Send: 0 [30] [20]
avrdude: Send: 0 [30] [20]
avrdude: Send: 0 [30] [20]
avrdude: Recv:
avrdude: stk500_getsync(): not in sync: resp=0x00
avrdude done. Thank you.