I have something close right now - it's a tiny leather bag just big enough to hold my pda in its aluminum case and my cellphone, on a thin, removable leather strap. It cost me $6 at one of the leather goods stands at a rest stop. HOWEVER.
However, the strap is the wrong length. When I wear it across my body, it ends just above where my waist would be if I had one, which is awkward and not attractive.
The ideal bag would be about that size, with an adjustable strap - when fully extended, it would be long enough to reach my opposite hip, which would place the pda where it would be if it were clipped to a belt. It could also be adjusted to wear as a waistpack.
Anyone know where I can find adjustable straps?