I also have the Sony a100.
Bring it with me on nearly every cache.
I have two bags, both by LowePro:
Inverse 200 AW, primarily for hiking.
and the CompuTrekker AW (this rarely has the laptop while hiking... but it makes a great way to travel with camera and laptop).
I also have a very cheap looking, although insanely functional camera box/bag. Hard sides, hold the camera with lens, spare lens, and room for batteries, memory, etc. Small, carries well.
In short... you can't catch fish if your pole is in the truck. Same holds true with your camera, ain't gunna get many pictures if it isn't accessible.
Good luck!