Looks like I picked up a rock on the very leading edge of my hood. The ding is small but it looks like it went to bare metal and is already a nice dark brown...rust? I don't want the rust (if that is what it really is) to get under the rest of the paint and cause any poblems. Any suggestions would be welcome as to how to take care of this.

If you're sure it's rust start with steps 1 and 2; if it's not rust but primer, go to step 3:
1) get a dremel, small cone grinding stone;
2) clean off rust to shiny metal;
3) use needle to pick edges of chip to get the loose paint off;
4) clean area with rubbing alcohol (let dry);
5) apply dab of touch up paint (let dry about 1 hr);
6) repeat above step twice;
7) use credit card as a chisel / spackle knife and scrape off excess touch up from surrounding paint (kind of like filling a nail hole in a wall before painting);
8) OPTIONAL - use clay bar to smooth whole thing over;
9) let dry overnight;
10) wax (firm pressure over area).

Seems long, but pretty quick and with practice it works well.

Sorry to hear that. I haven't had to do this on my X but I have on other vehicles in the past. For bare metal dings I use a pen-like devise I got a few years ago in the local auto place. It has a tiny wire brush on one end and a rust inhibitor/neutralizer tip on the other (works like a marker). For the repair itself I took matching color (Dupi-Color) and dabbed in a thin coat with a toothpick and let it dry per instructions for multiple layers. I repeated this until the successive layers came slightly proud of the surrounding area and then carefully shave the repair down with a razor blade. I let it dry before blending it all in with polishing compound on a clean rag. The final act was ended with a three-step wax session of the area. There is scratch filler/primer available from Dupli-Color but I've never used it. I've also never used the combination paint/clear coat touch-up as everything blended nicely after the polishing and waxing and I have this wierd feeling that it won't turn out right although I might experiment in the future. Good luck!!