I think that very few Chinese martial arts have developed forms for knife fighting (see below for Ted Mancuso's statement), but this doesn't mean that they don't have knife fighting, as an extension of their body art, although as has been pointed ou here, for cultural reasons it would never be the first choice, so it wouldn't be a priority.
What do you want out of knife-fighting training? If you really want to learn how to fight with a knife, pick a specialized art, such as Silat or a Filipino style. I'm sure other arts have it but I can't remember (Savate has an extensive curriculum for razors, or Folders, don't know the name in english).
Otherwise, some teachers have developed modern forms for the knife that take advantage of the body motion that a practitioner has developed in his art. This dude developed a knife form for a style of Xingyi that would be good for a Taiji player, since it's more "soft and rounded" than your typical Xingyi. I have it and it's OK, certainly learnable from video, and adaptable to the type of motion a Taiji player may have developed:
http://plumpub.com/sales/vcd4/coll_XYdai2.htm(Dai Xin Yi Coiling Sword. Double VCD , See Main VCD page for prices, CHINESE ONLY
From Ted Mancuso: "Odd's Bodkin! And that's what we have here. A very thin half length straight knife. This is, as the full name suggests, essentially a piercing dagger. About nine inches long. Actually a pretty interesting set. Though we much appreciate short weapons we want to take a minute to explain that people often ask about them, thinkng they are "more practical." Chinese martial arts has a very different position from, say, Okinawan arts. They believe that short weapons are great and all that but hardly need to be taught because, being short enough, they are simply extensions of normal movement. Thus a Xin Yi practitioner should be able to immediately adapt any short weapon. Of course this is true but it's awfully intersting to see the formal intepretation and also to possibly pick up some special movements ancient masters though germane to this weapon.. "). I would add that it may be a piercing dagger, but it also probably cuts well because a lot of motion in the form is about circular "slicing" patterns.
Erle Montaigue has a set of 12 drills (or short-short forms) that are not bad either, and resemble some styles of Silat that emphasize explosive sudden patterns of knife motion. They would certainly be a good thing to add to any arsenal. Finally, some Silat guys have actual "forms" that you could work with to learn stuff, although your body motion might be different from theirs, but at least from a technique point of view it would be something to start with.
I can recommend this one, although it's about the Kriss (different way of holding the knife and so on), not exceptional but good. It includes simple, introductory forms, that could be adapted by a competent Taiji player as the start of some exploration of Taiji knife.
http://www.saudara-kaum.fr/product_info ... cts_id=108(L'Ame du Kriss, Soul of the Kriss, has english dubbing in it)
Finally, I would add that I do not think this statement ("... You do not need the whole force of the body behind a knife. If you sharpen it properly, it will kill with very little force. So taiji, which is about whole body force, is irrelevant and maybe even a hindrance.
...") is very accurate. Taiji is also a lot about sensitivity, ability to develop a relationship with your opponent's motion and "sticking" to him. These would all be very good qualities to pick up in a knife "fight", God forbid I ever get into one. All the circular motions of the body and hands would fit very well with a knife method.