Firstly, responsivizing elementaryos.org is a great initiative and I think this mock-up is a nice starting point!
However, I'd like to offer a few humble points of criticism: I have to agree with whuskyw in that the black background is random and not in line with the desktop version of the site.
While the white space is nice on the login and front page, the Journal feels a bit crowded. What would help, I believe, is separating the posts like the categories are separated on the front page -- putting the time of posting below the title (and/or perhaps together with the number of comments) and picking another color than one indicating that this particular text is a link. One could then put the post's tags below the title to save space and in that process make the headline more prominent.
Also, search is a nice feature, but in my eyes, the space that it currently occupies would be better used by a main navigation bar. You could put that above the search or whatever, but I believe that if the search doesn't use AJAX, it isn't nifty enough to take up that amount of space on such small screens.
Looks fine, but the black background... The desktopversion has a bright one and I think both versions should match, don't they? It would be great when there is such a winterboardtheme (systemwide theme) for jailbroken devices But I wonder if this is really necessary? When I open the webpage I'm reading the journal and this is already in a well form and design.