Thats weird.
and Shotwell is present on the 'new' system but not on this one.
when you upgrade system, it does not necessarily install new packages: I have Squeeze system I upgraded from etch install, e.g. software-center is not presented.
what am I trying to say is that, when you upgrade system (no matter is that testing to stable, or stable to testing upgrade), upgrade process just install new versions of already installed system, and does not pull new software packages, unless it is absolutely needed.
In your case, when you installed squeeze, shotwell was not part of newly installed system, upgrade or dist-upgrade won't pull it. dist-upgrade will just.... hm, upgrade
Regards
------------
Roman