[R] transform large matrix into list

[Stefan Mischke]

Dear List

I need to transform a large matrix M with many NAs into a list L with 
one row for each non missing cell. Every row should contain the cell 
value in the first column, and its coordinates of the matrix in column 
2 and 3.

M:
	x1	x2
y1	1	2
y2	4	5
y3	7	8

L:
v	x	y
1	1	1
4	1	2
7	1	2
2	2	1
5	2	2
8	2	3

I'm trying to do this with a loop, but since my matrix is quite large 
(around 10k^2) this just takes a very long time.
There must be a more efficient and elegant way to do this. Any hints?

Thanks,
Stefan