Lowepro is excellent for outdoor use - I have about 8-9 of their different bags for my camera gear.
Make sure you get a bag with the AW (All Weather label). The fabric itself is made quite water-resistant, but all AW bags have a built-in waterproof rain cover which keeps water out even in a torrential downpour.
And Lowepro's customer service is top-notch. Last year, while shooting the Pushkar Camel Fair, my beltpack kept coming loose repeatedly and fell off a couple of times. I shot off a really annoyed email to them - one of their VPs personally Fedexed me replacement buckles (at a cost of over $100 to them) and one of their design guys emailed me about some suggestions I had made re. the design. Hard to beat that.
THAT being said, I am considering being unfaithful to Lowepro - this particular bag has really caught my eye. It is a camera bag, but looks like it would be ideal for carrying a bino, field guide, notebook and various bits and bobs:
http://www.thinktankphoto.com/ttp_ChngUp_Configs.php
http://www.thinktankphoto.com/ttp_product_UrbnDsgs.php
Vandit