Update 2011-12
I left my sweet dreams about open source mobile phone for some time.
Since Nokia N900 was released I was its happy user (and I'm still using it) but I came to conclusion that mobile phone is better not to be a same device as mini-notebook or camera or anything fragile and hungry for battery.
I end up buying tough Sonim XP3 to use it as phone only so I'm charging it only once a week and have no worries about losing connectivity due to lack of charge or wet weather or whatever.
Apple iPhone? You must be kidding.Windows Phone 7? Come on, Microsoft incompatible with ethics.Android? No, thanks.- Perhaps
Nokia's Maemo? Probably not, after their alliance with Microsoft.
Openmoko FreeRunner - yes, that's the one.