Bug 361631

Search via Google reveals other instances of this problem.  See
http://www.google.ca/linux?restrict=linux&q=%22%5Bsnapscan%5D%22+%22warming+up%22
https://bugs.launchpad.net/ubuntu/+source/linux-source-2.6.20/+bug/93515
Apparently the issue is really with libusb.

One workaround suggestion was to add 'scanbuttond' to the mix (which
has a slew troubles of its own - see bug 383131). That apparently
did have a salutory effect on F7 but made the situation much worse
for F8 (where 'snapscan' looks like an improved thing on its own).
'scanbuttond' was used on an installation in question only for that
side-effect and not really to service any button events.
 
All the above is a "seconhand knowledge".  I do not have a scanner
using 'snapscan' driver.  I was just trying to help my friend which,
unfortunately, interacts with computers only in "just a naive user"
mode.

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With Fedora 10 version of sane-backends (sane-backends-1.0.19-12.fc10) the problem (and the patch) disappeared.