Think Tank Photo do a holder called a Digital Holster and also a separate chest harness that is discreet enough to wear under your main pack without feeling like the two sets of straps get in the way.
The whole lot comes to £50 and you can adjust the strap lengths above & below the bag to get it sat exactly where you want it. It also pulls in nice & tight if you want so it doesn't get bounced around all over the place on rocky sections.
I've got a D80 with the 18-135 lens and it just fits in the Digital Holster 10 that I bought. The nose of the bag extends if required and it will take my 70-300VR as well. If I was buying again I'd probably get the next size up though for a D80 size body (as it is quite snug).
It comes with a shoulder strap for normal use and a waterproof cover, which are handy additions.
Here's a pic from their website:
and a link to the main Digital Holster page: http://www.snapperstuff.com/photographyproducts/categories/Digital%20Holster/listproducts.asp