$ wvunpack -o . "foo: bar.wv"
WVUNPACK Hybrid Lossless Audio Decompressor Linux Version 4.60.1
Copyright (c) 1998 - 2009 Conifer Software. All Rights Reserved.
restored ./ bar.wav in 3.80 secs (lossless, 32.84%)
Looks like wvunpack doesn't like ':' in filenames when using the -o switch with a directory.
Oops, I had some Windows-specific code left over in the non-windows case (in Window's filenames, obviously, the colon has a special meaning).
Fixed in SVN.