I've been bouncing around to a few threads on this exact issue for two days.  I've only been using Ubuntu on my HP Pavillion dv2116wm laptop for a few weeks now, but I was really trying to fix this issue.  I'm not really good at the coding and understanding some of the technical jargon on the threads here, but after a bit of monkeying around I was able to successfully get my scanner to work repeatedly without too much trouble.  So it may be a bit off topic from the current angles being worked on, but maybe it'll help some of you guys, I dunno.  I'm running Feisty Fawn on my HP (a recent fresh install from Vista), and the scanner is a Canoscan LiDE 30, which seems to be a pretty common scanner or similar to most of them.  Here's what I did in lamens terms, and it works great.

I loaded terminal and just ran su to be root.
I installed scanbuttond simply by doing an  apt-get install scanbuttond
Then I ran scanimage -L just to make sure my scanner was being recognized by the pc....and it was, I saw    plustek:libusb:002:004' is a Canon N1240U/LiDE30 USB flatbed scanner
From here I just typed gimp, and launched gimp
In gimp I did the  File>Acquire>Xsane>plustek:libusb:002:004 option (xsane also shows my built in vid cam on the laptop, but that's still not working...separate issue)
At this point, Gimp barked at me saying I shouldn't be running as root because I could screw something up...but I trudged along anyways
It launched Xsane with it's multiple windows
I hit Preview in the scan preview window, and instead of giving me an error...it started scanning!
I'm able to preview multiple times, use Xsane features to change file names, sizes, etc, and all the scanning options seem to work.  I scanned multiple images!

I jumped all over the house.  
Hopefully someone here can use this little bit of info I seem to have fallen into.  Like I said, coding in Linux isn't my greatest talent, so I tried to find a workaround that was both easy and fast, without writing any code.  Thus far, there's only one catch.

Once you restart the computer, the scanner is dead again.  It's back to only being picked up by the pc, but not scanning.  Gives me an error, even when I launch gimp as root and try to Acquire from the scanner.  So I simply ran
apt-get remove scanbuttonb
apt-get install scanbuttonb

Launched gimp again, everything working gloriously again.  So while everyone's working on the 'official' fix or at least a correct way of doing it, maybe this will help those of you stranded with limited Linux knowledge and just a need to use your scanner real quick.  I'll fart around with it some more the next few days, see if I can figure out some sort of fix where you don't have to reinstall anything each time you reboot.  I'm not sure if the USB issues with scanners and the fact that my HP laptop loses it's mind when it goes to sleep or is put in hibernate, but it may be.  That feature does nothing but give me a black screen when I wake it up, and I have to force it off and back on to regain access again.  So IF Ubuntu caused the USB scanner issue in the upgrade to Feisty Fawn simply due to including a sleep/hibernate feature for us laptop users....I seem to have gotten the raw deal on that, because it still doesn't work AND it screwed up my scanner, ha ha.

Anyways, run it, lemme know what you think.  Suggestions, advice, feel free to own me, I'm new to the forums, new to the troubleshooting, still trying some of your fixes!
Thanks!