I tested the latest Opera release[1] on Windows, and it has full support for the Web Storage and Web Database API's. In order to support this browser you don't need to download a new release of any gwt-html5-database or gwt-html5-storage libraries (at least as far as I can tell after a short testrun).
One note of caution, though: The current beta (10.50b1) has a non-functional localStorage, which is a bug. According to Bruce Lawson this bug is already fixed[2] in an internal build, the next release will reflect that[3].
FYI, Cheers,
Bart Guijt
E: bgu...@gmail.com
T: +31 6 30408987
[1] Opera 10.50b1 release: http://www.opera.com/docs/changelogs/windows/1050b1/
[2] http://twitter.com/brucel/status/9376997491
[3] http://twitter.com/brucel/status/9378836616