I have an HP CM1312nfi MFP multi-function colour printer, fax and scanner. xsane worked fine in 9.04. The first time I tried to scan after upgrading to Ubuntu 10.04 it didn't work anymore. (Breaking it may have been something I did, rather than the upgrade itself.)

First xsane told me that it couldn't find any devices. I reinstalled all the hplip and xsane packages, and that got me a message that xsane couldn't open the device -- giving a name that was obvious that it knew about my scanner.

I found a message in /var/log/syslog that xsane couldn't find the file "/usr/share/hplip/scan/plugins/bb_soapht.so". So I ran:
sudo hp-plugin
and answered the question about license. Then xsane (and the new Simple Scan) worked.