What are you guys using under your Bivy sacks for protection? (I'm thinking if a newbie reads this post they are going to wonder just what in the world I'm asking! LOL)
I just purchased a TI Goat Bivy to try out and was thinking I will need to throw something down on the groud to help prevent tears, etc.

I don't use anything, but then the floor on my ID salathe was 200d and my new ID micro bivy is 70d nylon, not silnylon. The issue with tears/wanting to use a groundsheet is the very reason I have yet to invest in an ultralight bivy sack. I'm sure I will someday however, in which case I'll likely use an old mountain hardwear tent footprint I got for $0.83 at an REI scratch and dent sale. I have also used it as a tarp and a place to put boned out meat to keep it clean. I am beginning to learn that multi-function gear can save a lot of weight and keep things simple. I highly suggest using something for a groundsheet that you already carry or could use for other important tasks.

I have used a piece of tyvek material under my Ti goat bivy. The tyvek can also be used like Snowy mentioned for meat or a small tarp.

I was thinking about using the footprint from my BD Lighthouse, but have read where lots of people seem to like Tyvek, but I'm wondering how packable it is.

Fox: After it's used some it gets more workable. I've seen some folks fold it like a map but I roll mine up tight and secure it with a long piece of paracord so I have tarp pitching materials handy. I like it because it's tough and cheap.

When going lite I use my 9 oz bivy as the ground cloth.

I use the polycryo ground sheets from Gossamer Gear under my sleeping pad in my tarp tent. It is 40x96", weighs 1.5oz and packs down to nothing. Durability might be an issue, but they only run $3 each. I only have 3 nights on mine so far, but no holes or tears yet.

http://www.gossamergear.com/cgi-bin/gossamergear/polycryo_ground_cloth.html