I notice that the new version of iptableslog (0.9) is set up to retrieve the timestamp of packets from a new field "local_time". This is a unix timestamp stored as an INT.
I don't know about other versions of ulogd, but using ulogd v1.23 with the mysql plugin on CentOS 5 there is no "local_time" field. However, the same data is stored by ulogd in the field "oob_time_sec".
To get timestamps working with iptableslog 0.9, I simply performed a search and replace in the iptableslog directory, replacing "local_time" with "oob_time_sec".
If this is the usual method of logging timestamps for ulogd then perhaps this can be incorporated into a later release?