Good question. Walker complained about a sore elbow yesterday - with anybody else, this wouldn't really be worth worrying about, but with Walker, every health-related issue is worth following. I doubt this is anything terribly serious, but it bears watching.
Usually, a club won't announce whether a player will get a day off until an hour or so before the game (there are exceptions, of course). If you can make late lineup changes, go to a site such as yahoo sports, click on the game your player is in, and check the starters (which will be available anywhere from an hour to twenty minutes before game time). This won't help with truly last-minute scratches, but would let you filter out Walker's off days.
Giambi is hard to figure out. I think he'll heat up soon, but you can't overlook the fact that he always put up better stats in Oakland than on the road. give him a bit of time to adjust - he'll be fine.