Portal Tunneling
I'd say tunneling was intended, as they gave the portals a travel time. If the devs didn't want this effect, they would have given the portals instant, or very fast, travel time.
Also, as was mentioned, tunelling is needed (i think) to get the least portals on chamber 15.
I definitely agree with Korjagun. To me, portal tunneling feels strongly like exploiting a glitch -- cheating. I think that Valve is aware of it, but I don't think they really intended this move to be part of standard Portal repetoire. My guess is they had to leave it in because fixing it would break the passing-through-portals physics in lots of other situations. But that's just speculation.
What I really have against it is this:
1. It is a simple trick to learn and use almost anywhere there are two portalable walls.
2. It can be used to bypass puzzles entirely, or even multiple puzzles at once.
I'm all for tricks or shortcuts that take some insight and some skill, but portal tunneling is dead easy and can be abused to no end. And if you're repeatedly using the same trick to bypass most of a level's puzzles, why are you even playing the game? Like Korjagun mentioned, you might as well noclip to the end of the map and say you beat it. 
What maps can you skip entire puzzles using this?
To remove a player's ability to tunnel simply force them to put a portal on a floor or ceiling to move on.
it can't be a glitch, unintended but not a glitch, If you think about portals in a real life setting you would be able to stand on the gap in between them becuse you could curl your feet over either side, it would probably hurt but it would be do able. Then you grasp the wall, pull yourself out, shoot another portal and swing yourself back in. It would be possible in real life if portals were real.
does anyone have any youtube examples of tunneling to be used to skip puzzles? with the exception of test 15
If tunneling is the goal of the puzzle it will encourage players to try tunneling in other areas of the map, so use it carefully so they dont tunnel through the whole thing.
First off, thanks for all the replies.
I have to say I am little surprised that a number of you consider tunneling to be a glitch/exploit when it is required in order to complete challenge 15's fewest portals.
At the same time, I do understand that tunneling can be destructive to a puzzle, but it only takes a bit of planning and testing to prevent it's use from becoming an unintended solution (requiring a portal on the floor or ceiling is an easy way).
The only places in the game that I can think of where you can use tunneling to puzzle-skip are the chamber 15 platform tunnel and the chamber 19 platform tunnel (the one that goes into the fire pit).
Right now I'm sitting on 4 increasingly difficult portal-tunneling puzzles (a chamber 15 style puzzle being easiest) and will try to decide which one I should put in my map with the understanding that I may need to pull the puzzle altogether if there is a lot of negative feedback once the map is released as a W.I.P.
I'm just not sure where I want to draw the line between challenging and mind-bending.
I think most importantly, you ought to make it clear to the player that the puzzle requires use of a technique that is not obvious and not part of the "standard" (by which I mean non-challenge) Portal repertoire. That and try to avoid things like abusing collision detection bugs or physics glitches...
indeed, make a new overlay sign (like the one for box dropper or flinging) but for tunneling
Ironically,
To me though, the important difference is making the player discover the solution by thinking about how to solve it, rather than defaulting to "Look at me, I'm standing in a wall! Hey, now look at me, I'm still standing in a wall!" In the above mentioned example, I think even those familiar with portal tunneling have to sit and think, because a) it's different from the usual vertical-surface tunneling, and b) it's trickier when you're fighting gravity.
Shmitz, I must have played Accident Prone 4 or 5 times and I do not remember needing to do that.
I agree with you though that those little descriptor signs detract a lot from a puzzle and should only be used if you are making a puzzle that is strictly a tutorial.
There is really only one way to avoid players tunneling, and that's designing the puzzle specifically for it not to be possible or of any use to the player. Take Post-Portal beta 1 for example.