The reason for the error is that nvnetlib.o in the nvnet package is compiled with GCC 2.96 and mixing GCC 2.9 objects with a GCC 3.2x compiled kernel is known to cause major stability problems. The latest version (261) of the drivers works around this warning from modprobe as I explained in my previous post, but it doesn't fix the stability problems.
Last time I tried I was unsuccessful in getting the version 261 drivers to work on Slackware 9.1 (even with a custom built kernel) without having them lock up the system under the slightest load. Using an older version of the drivers seemed to help, but the same kernel configuration on a Slackware 8.1 base system was rock solid, so it may have been a matter of time before I found another situation that would cause 9.1 to lock up.
In the end I got sick of all the stuffing around and trashed the system in favour of FreeBSD using a port of the nvnet driver and it has been much faster and more stable ever since.