Hi, I'm still surprised by the lack of a simple SDL_ResizeVideo function allowing to dynamically resize the window (under Windows, under other OSs I dunno) without loosing the current OpenGL context. So I simply added a function to SDL and it works! Why not implementing something similar in the official release? Here my perhaps naive solution, but perfectly working on every PC I tested so far (quite a lot of different HW configurations):

/*
 * Update only the size of the window, without killing any context
 */
void SDL_ResizeVideoMode(int width, int height)
{
   SDL_VideoDevice *video = current_video;	
   video->screen->h = height;
   video->screen->w = width;

	/* Reset the mouse cursor and grab for new video mode */
	video->UpdateMouse(video);	
}

I just have to call that when I receive a SDL_RESIZE event, after I modified the  glViewport. 

Dunno if this is a dirty and ugly solution for you, but it always worked for me and every time I update my SDL lib, I've to modify the SDL source code just to add this func. Pls let me know if there's a better way to get that and don't hesistate to tell me that I'm a dumb wheel reinventor! ;)