bejustice, this definition can be confusing when applied to tomatoes. To help clarify, imagine the following scenario:
You have a tomato plant with red fruit. A somatic mutation creates one branch with yellow fruit.
Now, if you save seeds from one of the red fruit, they will NOT produce plants the same as the parent - i.e., with all red fruit except for one branch of yellow fruit. (the yellow-fruited branch is not inherited) However, if you save seeds from one of the yellow fruit (a point beyond the somatic mutation) all of the grow outs will produce only yellow fruit. So, technically, the yellow fruit is not inherited from the source (parent) but IS inherited if seeds are saved from a growth point beyond the mutation point.
In your case, if an entire plant produced the original pink fruit (shown in your photo) then it is most likely a crossed seed, or a mutation in the seed, and not a somatic mutation (it would have to mutate at, or near, the base of the plant to be a somatic mutation). However, if only one branch produced the pink fruit then it is a somatic mutation and seeds from the pink fruit will produce plants with all pink fruit. I am only personally familiar with one other somatic mutation: Casino -> Casino Chips, and I don't think the form/color has changed since the original mutation. I don't know if this is true for all somatic mutations in tomatoes.
At any rate, you should grow it out to check for stability. Also, you might start another thread to ask if anyone is aware of this mutation occurring previously - you don't want to rename something that is already available!. Carolyn, or one of the other experts will know and chime in. Congratulations on your find, and good luck!
Steve