My TL 04 4-speed transmission is suddenly clunking into reverse when cold in the morning. Fluid seems OK. Seemed to happen after a battery went flat, but nor sure. Forward gears fine.

If battery was replaced the the TCU/ECU needs to relearn. In the mean time, either full depress the brake pedal before shifting to R or give the accelerator a tap to reduce the idle rev and then shift to R. I would still fully depress the brake. It happened to my old KW on the two occasions I changed batteries.

Interesting, i've been putting up with this for a while in my TL AWD, but in my case the TCU isnt the cause.
Re-learn procedure here http://www.aussiemagna.com/forums/showthread.php?t=86194&highlight=transmission+relearn+procedure
Btw, it should be a 5 speed, not 4?

Btw, it should be a 5 speed, not 4?

Not necessarily. TLs still had 4-speeds in ES and LS models.