Where does it come from or how is it chosen over regular Maple Necks - is there a tonal difference?
Birdseye maple is a cut of wood from one of several types of hard maple, most commonly Acer Saccharum. The only real difference between birdseye and regular hard maple is the figure, which doesn't really do anything to the tone of the wood. It's still as bright as any other type of maple. Scientists really have no clue as to why the figure occurs, but we guitarists are sure glad it does ;).
I would simply consider it an aesthetic benefit--and what a benefit! Under gloss it looks beautiful!
The name "maple" for wood is very ambiguous (there are many subspecies and also other species sold under this name). If one stays with the "commercial" names, on an aesthetic level there is plain maple, flamed maple, quilted maple and birdseye maple.
It can have really complex figures that reveal themselves fully after varnish. The effect and shapes are very diverse from one example to the other. Here is an example of a guitar neck (found at 1.bp.blogspot.com):
Plain maple can be very nice in its simplicity (image taken from the Gibson Website).
As jduv said, there is probably no impact on sound but it may require special care when carving and finishing.
There are so many things to look for such as the quality of the neck shape itself, hardness, drying process, etc. as explained in other questions here.