Could this be the infamous Pluot?? A cross of a plum and apricot. There are at least four types in various colors of skin and flesh.

I doubt that this is a pluot. I grow three types of these (from Zaiger Genetics, purchased through Raintree Nursery). It is not like these. The mystery fruit is much smaller, the size of a quarter, as shown in the photo. It also more productive and earlier than my standard pluots.
-Mark

Looks more related to the plum. I have a honeymoor plum which is the same size,but has slight blush and is sweet.

A friend from the NAFEX group I think has correctly identified it as an apricot-like cherry plum, Prunus cerasifera. Here is a photo of such a plant for comparison.
{{gwi:668259}}

I couldn't see my original photo for a time today, so I have included a link to it if anyone else has problems seeing it.
-Mark

Here is a link that might be useful: {{gwi:668256}}

plum cherry

mirabella
http://en.wikipedia.org/wiki/Mirabelle_plum