I am having the exact same issue with Debian 3 and a 2.6.2 kernel I compiled myself.
No swap space is configured according to /proc/meminfo
/etc/fstab has the proper swap listing
swapon /dev/hda4
swapon: /dev/hda4: Invalid argument
swapon -a
swapon: /dev/hda4: Invalid argument
That is the correct syntax according to the man page.
swapon is a utility included with my 'mount' package on debian. The version is 2.12-3 in the testing fork. There is no newer version available as a package.
According to rpmfind.net there is no newer version of mount than 2.12.
***FIX***
dmesg shows the following error message:
version 0 swap is no longer supported. Use mkswap -v1 /dev/hda4
***EXECUTE***
mkswap -v1 /dev/hda4
swapon -a
cat /proc/meminfo
Replace /dev/hda4 with your swap partition, viewable in /etc/fstab
Everything working well now!
Figured it out while typing this out.