I've been reading up on the Sigma 15-30mm, it seems a good lense. As to the protection. I would think your on about a UV filter for the front element?.
'Some Sigma dealers advertise the lens as taking 82mm filters. This is misleading. The lens cap consists of two pieces, a ring and a cap to the ring. The front of the ring has 82mm threads, and if you're using a digital SLR and aren't zoomed out to 15mm, you can leave the ring on the lens and screw a filter into it. But vignetting occurs even on the D1 at the widest focal lengths, and at virtually all focal lengths on a 35mm body if you put a filter on the front. (There is also a small "slot" at the rear that can be used to slip in thin gelatin filters cut to size.)' This it taken from Thom Hogans reviews.
It would seem that to protect the glass without resulting in much vignetting you would have to acquire the both pieces (which i have not come accross yet).
I shall keep looking