Hello Gumbyjan,
Sounds like you have experienced one of the known problems that come with your 10.7 software update only, here is what happens:
After departure the climb page goes in single engine mode, after some basic conditions for this to happen have been met:
Thrust lever position difference exceeds 52 degrees and ground speed greater than 30 knots.
If you get this message in the air, simply have the PM press the LSK 4L button and execute to get back into dual engine mode
...after you have seen it once a non-event really
Now, i'm sure you are puzzled about the basic condion, but I suspect your company allows for single engine taxi. It could be the previous crew have exceeded the taxispeed of 30 knots (and the airplane electics have not been powered down) before you took the aircraft from them.
To me it may even be they did not exceed that taxispeed or have had the difference in thrustlever position, since it is an anomaly in the software.
I did not cook all this up myself; it's all in the Boeing FCOM bulletin dated june 11, 2007 (your operator should have distributed this to you).
Safe flights,