Sounds exactly like what was going on with my '10 K1300GT. After fighting all of '10 and most of '11 with the shop and BMW it was finally determined that the software update didn't help and it wasn't due to poor quality fuel. Your local shop should have a service bulletin on this, I'll see if I can get the specifics from my shop, but in the end it was an adjustment to a servo (?) that was slightly out of adjustment, causing the fuel to be cut off.
The symptoms you describe are identical to mine... Driving along without a care in the world, pull in the clutch to down shift and ..... nothing. No warning bells, lights or anything unusual. The shop checked the computer and found no error codes either. It actually feels like someone just turned off the engine. I originally thought it might be the kill switch causing the problem, since there is no sputter or anything when it shuts down. My mechanic was actually at a BMW class during 2011 and they showed them at that time how to make this change. Typically this is one of those things that they tell the shops to NEVER EVER TOUCH UNDER PENALTY OF DEATH. However, in this case, it's what does the job.
So, since purchasing my '10 GT in early 2010...
1. Hot start problem, which they replaced the battery to 'fix' last summer, is now back.
2. Left switchgear failed in 2010. Replaced. No problems since.
3. Stalling for all of 2010 up until 2011 which resulting in one injury. Twisted my knee trying to keep the bike up. No problems since.
4. Cam chain noise. Replaced cam chain and placed guard in 2011. Replaced cam chain and sprocket this year.
I now have a little over 13,000 miles on this bike. It's been back and forth to the shop so much that I don't trust it. My previous bike, an '03 GT I put 60,000 miles on in the last 3 years... This bike has been disappointing. I'm about ready to sell it back to BMW!