If you're going after code conciseness, I would use this:
int[] result = new int[8];
byte leafposition = 42;
int arrayPosition = 0;
for (int iteration = 0; iteration < 8; ++iteration)
if ((leafposition & (1 << iteration)) != 0)
result[arrayPosition++] = iteration + 1; // one-indexed
If you're going after performance, I would use a pre-populated array (of 256 entries). You can either generate this statically (at compile-time) or lazily (before calling your method the first time).
int[][] leaves =
{
/* 00000000 */ new int[] { },
/* 00000001 */ new int[] { 1 },
/* 00000010 */ new int[] { 2 },
/* 00000011 */ new int[] { 1, 2 },
/* 00000100 */ new int[] { 3 },
/* 00000101 */ new int[] { 1, 3 },
/* 00000110 */ new int[] { 2, 3 },
/* 00000111 */ new int[] { 1, 2, 3 },
/* 00001000 */ new int[] { 4 },
/* 00001001 */ new int[] { 1, 4 },
/* ... */
};
byte leafposition = 42;
int[] result = leaves[leafposition];
Edit: If you're using the lookup table and can afford a one-time initialization (that will be amortized through many subsequent uses), I would suggest creating it dynamically (rather than bloating your source code). Here's some sample code in LINQ; you can use the loop version instead.
int[][] leaves = new int[256][];
for (int i = 0; i < 256; ++i)
leaves[i] = Enumerable.Range(0, 8)
.Where(b => (i & (1 << b)) != 0)
.Select(b => b + 1)
.ToArray();