This may or may not help you, but it may provide some information to others reading this. IBS, colitis, and related disorders can be idiopathic in pets and can be pretty complicated to diagnose. There is a fair amount on the Web about that family of illnesses, and one fair source is the alt.med.veterinary news group; this is a link to a search for IBS on that group, and you can explore it further or post your questions there:
http://groups.google.com/groups?hl=en&l ... veterinary
Probably the first thing to do is stop the weight loss, which means getting food into the cat and keeping it there. I believe the Hill's product that dcluck mentioned is called i/d, and it's worth a try--you may have to hand/force feed the cat small portions several times a day. Depending on how far your cat has deteriorated, you might also consider a/d, which is a high density recovery product. If vomiting is a problem, metoclopramide is one medication that often is used to help with it. An oral digestive enzyme might help improve digestion/absorption of the food. I prefer to use it on the food before feeding and allow the food to pre-digest some, but if that won't work (if you hand feed, the enzyme breakdown can make the food too liquified to administer) feeding it with the food or giving it directly as a pill or solution is the option. If you haven't tried canned tuna or salmon, you might try that and see if it works. I'll take a guess and suggest the type packed in spring water; the oil-packed might be a little severe on the gut to start. If the cat will tolerate it without too much projectile vomiting or diarrhea, you can kick the daily energy intake by administering Nutri-Cal or a similar supplement available at pet supply stores. Spread a line on your finger and apply it to the roof of the cat's mouth.
There are several medications for treating bowel syndromes, and if your vet diagnosed IBS, then he/she probably already discussed them with you (and I assume has ruled out lymphoma). I believe the fairly new human medication for Crohn's disease has been adapted to animal use, but I'm at a loss to think of the name right now. What is used depends some on where the problem is--small intestine only, large intestine only, or both. Prednisone often is given, but it can have side effects probably none of which is worse than the disease. Although Primor (for dogs) is not recommended for cats, apparently because of the salicylate content, I believe it can be given if the doses are kept small enough. Individual animals can react differently to different medications, so it's important to monitor things. Unless the cat is allergic to it, I can't think of a reason why Nina's earlier comment about using rice water for diarrhea ( http://www.dirtdoctor.com/forum/viewtop ... 9e25bd9e9d
) wouldn't apply, so you might try that and see if it will help. You may need to administer it with a syringe. I also believe you can give the Gas-X type simethicone products for gas/bloat if that is a problem. Ultimately, the auto-immune character of the disease will have to be addressed; palliation probably will carry only so far. One of the problems is that another illness, even as simple as a cold, really can complicate matters for a patient with compromised digestion. If you want, PM me, tell me about where you are located, and I'll see if I can find some specialized veterinary references. You shouldn't need to go to A&M to get relief, and they may not be the best place to go anyway. One of my cat has a form of IBS, but he seems to have stabilized to a manageable point. I don't have all the answers, though.
As for herbal and/or holistic treatments, they very well may work or help, depending on what the problem is. If the animal has not deteriorated too far, you might have time to explore and test those approaches. You can search the Web for those therapies or possibly consult the books such as Dr. Pitcairn's holistic animal care book(s). I've seen statements that the Missing Link alone helped/cured at least one situation. If you hand/force feed or administer Nutri-Cal, you can add some Missing Link to whatever you are giving that way. Diagnosis and treatment may not be easy, but the disease likely is terminal if it is not treated. Good Luck.