Who knows why the capture command on the asa 8.0 release does capture packets but is not able to display (or export to wireshark) all packets.
The command I use (in several variations)
capture cap1 type raw-data buffer 33554432 interface outside circular-buffer headers-only [Capturing - 17124 bytes]
the command sh capture cap1
simply shows several lines (from some thousands)
Wireshark displays "The capture file appears to have been cut short in the middle of a packet"