Same thing here ... sort of. Also TH-46PZ850U. Almost 5 1/2 years old (bought new in April 2009, unit built in August 2008 IIRC), was working fine ... this past weekend suddenly wouldn't power up. Might've had only two blinks initially, but now it's always the 10 blinks on the red power light. Initially thought it was my (high-end) external power block/strip (because Ooma telophone box also was 'dead'), so I moved the plug to the other 'column' of outlets on the block ... powered up and ran fine. Next power up failed again with the 10 blinks.
From what I've been able to find online so far, it appears this is a capacitor failure on the MC201 and/or MC301 boards (which are located on the Main Power Supply board)? Some people have simply removed the capacitor all together and run problem-free for months. Others have removed the tiny 'dime-sized' board the capacitor sits on and soldered on a new IC chip and capacitor. Still others have just replaced the capacitor ... either by first removing the tiny board or soldering a replacement in-place (takes a steady hand and 15-watt, tiny point soldering iron)!!
Should I proceed along the lines of the failed capacitor, or are there other possibilities??