If the panniers that convert to a back pack are all like the ones I have seen, I'd skip them. If you are doing only day hikes, I have found that all I need is a shoulder strap for my handlebar bag. That works fine for all-day hikes if I keep it pretty light. Another option is to use a handlebar bag that converts to a fanny pack, Topeak makes one that is pretty nice if a bit small. You would probably want to add a bottle bag to the belt to carry water with that one.

If you are going to actually go backpacking, I'd do as Jamawani suggested. In that case I'd probably take one of the ultralights like the ones mentioned above (GoLite etc.).

If you are a trailer user with a BoB or similar, I'd consider just packing everything in a backpack and carrying it in the trailer.

Personally I am have thus far kept backpacking trips and bike trips separate. I tend to stick to hikes of a couple hours or less, with the exception being when we stayed a week in Yosemite. For me I think that is a better compromise, but if you want a more hybrid type trip it is certainly possible.