Why won't scanner show up for Scanner Sharing?
I also have a HP ScanJet 4890 and it has never worked natively with neither Snow Leopard nor Lion. The fixed communication to the HP software seems to be by design. As @kremalicious says, that also makes it not work with built-in tools as Image Capture or Scanner Sharing.

While I can live with that, it's quite annoying to wait more than a few months for new working HP drivers and software after a new OS X version is released.