Well ...im having the same issue Omegatron, the differenec with mine is that if I log in into the Forum first , then click on the photopost gallery link which I have on the forum, i get to stay logged in. After a while the session time sout and i have to go back into the firum to log in again. I noticed this recently ( last two weeks) and unfotunately i m not sure whether its was the Photopost or SMF that caused that problem but, i have always had it work well.
On the second part, you mention of Cookie path in both apps ( SMF does not have that setting to change- at least i looked around i could not find it). Same as the Cookie domain {When you say set it to .domain.com in both apps so cookie works with and without the www} do you mean that if my domain is kenyans.org, i should add the period behind ( ie: .kenyans.org) in photopost?
If you want to try it , take a second to set up an account on my integration on
http://www.kenyans.org.
Thanks.