Setting to texture to a camera I want to set up a portal effect like in the game Portal where one portal will lead to another, and the portals have the textures of what is seen from the other portal. If I have 2 cameras, and let the cameras be textures for the portals, I think it might work. Is there any way to make a texture be the view of a camera?

If not, are there any other approaches to this?
Your question doesn't tell us for which application you need this effect - a rendered animation I guess. (Or a game?)

If you don't need it to be live input, you can just do the following:  


Set your camera to the desired position. For every frame the camera is 'active' as the portal is deployed, you have to be in camera view. 
Now, just render the animation in this time period (as png's).  
Next, go into photoshop/gimp/whatever, and put the portal on it like a border. Everything outside it should be alpha. (To achieve an animated portal you should use different portal borders, a static portal looks quite boring...) 
Finally, back in blender, add a plane the size of your portal (you can also use the add-on "images as planes") at the place you want to let the portal appear.  


Animating the Texture:

As this is complicated to explain just take a look at this video, which explains the process in detail.

If what I said wasn't visual enough, I could help out with sceenshots and a blend file.