I was making huckleberry jam last night -- yes, huckleberries really do exist -- and needed to look them up on Wikipedia since I didn't have a recipe on hand and wanted to make sure it came out alright. While I was reading up on this strange fruit, something caught my eye. Apparently, the huckleberry came to be known as something tiny and insignificant culturally, and from there on to something reliable. The slang evolved from "a huckleberry to a persimmon," meaning the tiniest bit beyond what you could do or achieve, to "I'll be your huckleberry," which meant "I'm your man," or "I'm the one for the job." So, long story short, is this the true meaning behind the mysterious "my huckleberry friend" line in "Moon River"? I'm convinced! What do you think? You can read more about the evolution of the huckleberry as a cultural reference here, if you need more information.
The berries themselves, by the way, are very similar to blueberries but are more tart in flavour, and the jam came out perfectly. :)