Odd. Do you use the AOPA flight planner? Do you use DUAT?

Here is a snippet of the transaction log from trying it just now on a
VFR flight plan:

DUAT DOMESTIC;VFR;TAIL N15782;ACTYPE PA28/A;SPEED 110;DEPART
KPAO;ARRIVE KLVK;ETD 2003;FL 31
ERR228: ACTYPE:Invalid Aircraft Type.

And here is it working:
DUAT DOMESTIC;VFR;TAIL N15782;ACTYPE P28A/A;SPEED 110;DEPART
KPAO;ARRIVE KLVK;ETD 2003;FL 31
OK.
[ ... ]
Your Flight Plan will be transmitted to OAK FSS 1 hour and 56 minutes
from now for a proposed departure at 2003 on 06/
14/2005.

-- dave j