If you want to go the more civilian invisible route, then look at some of the REI offerings. Out in the woods, an assault pack fits in a little better, and is less noticed, but in an urban environment they stick out like a sore thumb. Depending on how you currently dress, and your job, where and when you are places, it might be worth really considering if you want to look geared up or not.
As much as certain oldschool rucksack style packs that are coming back into fashion have a very hipster vibe, they are invisible, and there are some on the market from very reputable companies.
My preference is 20-40L climbing style bags, like the lowe alpine crag attack. they ride well, have compression straps that let you tighten up the load, work really well for bulky items like cool weather gear, or the like, and are quite tough. There are many manufacturers that make a pack in this style, top loading, tall and skinny. Another urban invisible option would be one of the timbuk2 offerings, either a backpack or a messenger bag, custom ones are produced in the US (well, California, so close enough) looks like they have some deals running at the moment.
