Got my girlfriend to finally play and beat Portal in a single setting the other day, so I think I already have my co-op partner for that one, sorry 05 ;)
Of course, if it is anything like Portal, I am not going to mind tackling it multiple times (though, again, if it is anything like the first Portal, the very first time is a unique experience that can never be relived).
I am probably setting myself up for extreme disappointment, but I kind of have my fingers crossed hoping that Portal 2 brings us some hints that Half-Life 2: Episode 3/Half-Life 3 is on the way followed by full fledged confirmation at E3.
Come on 2011, don't disappoint me! At this point Half-Life feels a bit like a really good TV show canceled on a cliff hanger, one last installment providing closure to the current plot line and I could live content without another sequel.