Is there any truth behind a "leaky gut" diagnosis?

A “leaky gut” diagnosis is more based around the theory of a proposed mechanism of action of poorly digested proteins, for example casein, which provides opiate like peptides that are not completely broken down and are absorbed into the bloodstream through an abnormally permeable or ‘leaky’gut.
The peptides then cross the blood-brain-barrier and can interfere with neurological processes. 
Again theoretically, the ‘leaky gut’ could be caused by an enzyme deficiency or yeast overgrowth. Note that this is distinct from an IgE mediated allergic reaction or food chemical intolerance reaction. 

Further to what Rachel wrote, i.e. that much of this is theoretical, it should be noted that everyone most likely has a ‘leaky gut’ in that portions of or whole food components (proteins and other chemicals) are absorbed into the bloodstream. These can even be found in mother's milk, and could only have got there via the bloodstream. Not all food is broken down to the basic building blocks as taught in human biology. The process is not 100% efficient. The issue becomes what response a person's system has to the presence of these food components in the body.

To add, the leaky gut symptoms which are varied and very personal to the individual can become worse or activated during stress, a dose of gastro, consuming irritants to your body such as too much alcohol, caffeine, other chemcials found in food that irritate your system. 

To help the gut heal, it is suggested talking with an APD dietitian and discussing the foods you generally eat and the possible irriation for you, and your gut.  This could be done with an elimination diet, and testing the irritant, and your tolerable dose. 

It is not suggested to avoid foods because doing so ‘makes things better’, as you may run the risk of missing out on important vitamins, minerals, fibre and also enough proteins.  

Successful treatment includes several measures and includes food, stress management, probiotics and the like.  

Most people who wonder about leaky gut are wondering if they are not tolerating some food. As mentioned there may be several mechanisms that lead to the symptoms they have, and several ways that symptoms can worsen or lessen over time. The important idea is to find out exactly what foods may be causing the problems. Substances that often aggravate the symptoms include artificial colours, flavours and most preservatives, as well as natural chemicals such as salicylates, amines, and glutamates. Sometimes people need to attend to whole foods such as dairy or wheat as well.
It is wise to get the help of a dietitian to do what I call Diet Detective Work so you can work out a good diet to use for the four week diet trial, and then to guide you through test foods to find your own best diet. The good news is that with the new ideas available you will tolerate more than you think!

Leaky gut syndrome can arise from many causes, most commonly what is called dysbiosis, which means the gut flora are all out of balance; and severe leaky gut can be caused by people with acumulated mercury toxicity. The good news is that with modern pathology tests, clinicians don't have to make guesses. There is a specific specialised test using the urine called Intestinal Permeability, which is being offered by several pathology labs these days, which can confirm the provisional diagnoses of leaky gut.

“Leaky gut” is an intestinal barrier dysfunction. In order to absorb nutrients from our  food, our gut has to be somewhat “leaky” or permeable. The process of digestion begins when we smell or see food. Saliva begins the process and digestive enzymes break down the food so that nutrients may be obtained. In order to obtain those nutrients the gut mucosa “tight junctions” allow the digested nutrients to be absorbed into the bloodstream. A so-called “Leaky gut” occurs when these  “tight junctions” are forced apart allowing larger undigested particles to escape into the bloodstream. We are all individual as to the symptoms which arise from intestinal permeability. Food intolerances, stress, and heavy metal toxicity appear to be major contributors to “Leaky gut” .