I was just going to do another edit on my post to state that yes, I am running Win7 x64 with x64 Sigil 0.3.3 installed.
EDIT:
Did a test, since you stated that 0.3.2 installed to the Program Files (x86) regardless of being x64 or not. I did another uninstall/install but this time I installed to the Program Files (x86) and Sigil now appears in the 'Open with' list again. Going to test another theory but in the meantime...
EDIT2:
Think I found it, or at least it's working for me. I did another uninstall and reinstalled to 'Program Files' then did a search for Sigil in the registry and came across this:
I did a Modify and changed the 'Program Files (x86)' to 'Program Files' and Sigil now shows up in the "Open With" List. Seems that registry key is not getting changed or deleted.