I am using slackware-current and up until last week I was using fsarchiver 0.6.8 to do system partition backups using a command like:
fsarchiver savefs -A -a -z 3 -s 4300 /backuphost/xxxx-rootfs-201011182135.fsa /dev/sda2

Now I get an error:
backup cmd is: fsarchiver savefs -A -a -z 3 -s 4300 /backuphost/xxxx-rootfs-201011262135.fsa /dev/sda2
Archive will be split into volumes of 4508876800 bytes (4.20 GB)
filesys.c#204,generic_mount(): partition [/dev/sda2] cannot be mounted on [/tmp/fsa/20101126-213520-00] as [ext3] with options [user_xattr,acl]
oper_save.c#1032,filesystem_mount_partition(): can't detect and mount filesystem of partition [/dev/sda2], cannot continue.
removed /backuphost/xxx-rootfs-201011262135.fsa

I have upgraded to fsarchiver 0.6.10 using "SlackBuilds.org" fsarchiver scripts and repeated with no change at all to the error.

I booted from a systemrescuecd-x86-1.6.3.iso disk and used basically the same fsarchiver command and it worked fine.
The only change from 18/11/2010 - 26/11/2010 is the new slackware packages, which are actually quite extensive.

My Google searches have returned nothing useful.

my C and linux understanding is not good enough to enable me to fix fsarchiver even after it tells me the error occurs in filesys.c line 204.

Anyone using Slackware-current and fsarchiver know the answer?

Your problem appears to be that your /dev/sda2 partition cannot be mounted using the user_xattr,acl options.
I can mount an ext4 partition with these options on my -current setup.
Perhaps you have had a failed package update.
Try reinstalling the util-linux-ng-2.18-i486-1 package.

tried:
slapt-get --reinstall --install util-linux-ng
no errors
fsarchiver ...
same error
just for good measure - reboot, fsarchiver ..., same error
Same result on another computer also running slackware-current also with ext3 partitions.

You said it worked on ext4 so that got me thinking!
It seems to be ext3 related as I just tried fsarchiver on an ntfs partition
and did not get the same error.
Humm, maybe it is time to change from ext3 to ext4. Time to experiment on my test computer.
I hope I am not the only one left using slackware on ext3
It would be be better if ext3 was supported.

Just converted the root partition from ext3 to ext4
fsarchiver ... Same error !!!

What was your fsarchiver command that worked on ext4?

I do not use fsarchiver, although it looks like an interesting program.
My reply was based on the error message in your first post. I have a hunch that there is something wrong with your setup that is causing a problem with the mounting of /dev/sda2.
Can you try mounting an ext3 or ext4 partition using the user_xattr and acl mount options from the Slackware setup where you are attempting to run fsarchiver?
i.e. Can you successfully use 'mount -o defaults,user_xattr,acl /dev/<some_ext3/4_filesystem> /mnt/tmp' to mount an unmounted partition?

I maintain fsarchiver script on SBo. I just tried fsarchiver on ext3 partition and it works perfectly. I'm using slackware (x86) -current with latest updates. My fstab entry for ext3 partition is below:

Have successfully mounted using:
mount -o defaults,user_xattr,acl /dev/sdc1 /mnt
ls -la
...
no problem.

Can also do fsarchiver using:
fsarchiver savefs -A -a -s 4300 /data2/mytest.fsa /dev/sdc1
where
/dev/sda2 - root filesystem (/) first disk
/dev/sdb1 - /data2 (second disk)
/dev/sdc1 - /data3 third disk

but get the error when doing:
fsarchiver savefs -A -a -z 3 -s 4300 /backuphost/xxxx-rootfs.fsa /dev/sda2
where
/dev/sda2 - root filesystem (/) first disk
/backuphost - nfs mount filesystem

Is the problem that /tmp is part of / (root file system) which is what is being archived?
If so how do I get fsarchiver to use something other than /tmp?
and if a computer only has one disk and one partition (/) what do I use then?

You are trying to mount your entire active filesystem onto /tmp.
I do not like that idea. I would stick with the LiveCD method that you know works.