The Street Skin for iPhone 4 is made of TPU, a shock absorbing semi-flexible rubber, which encases the iPhone leaving only the screen exposed.
Along the sides there are cut-outs for the volume buttons and mute switch, while the top and bottom have holes for the power button, headphones port, dock connector and the speakers. On the back, the camera and LED flash are left exposed for snapping pictures.
The case is about 2.5 mm thick, making it pretty slimline for a rubber case and allowing it to fit in most non-custom docks without having to remove the case. It wraps around the front of the iPhone 4 as well, meaning that when laid flat on a table the screen is kept off the deck preventing scratches.
The TPU gives the case a grippy texture, but thankfully not enough to cause severe pocket torsion, where you simply can't get the iPhone out of your pocket due to friction. The rubber itself has a distinctive crisscross tire tread pattern that makes it stand out from the usual cheap silicone-based fodder you see on eBay (check out the gallery below to see it in its full glory). The corners are pretty well protected, which should help if, heaven forbid, you manage to drop your precious glass iPhone. I wouldn't advocate that the Street Skin gives great impact protection, but with a bit of luck the TPU should provide enough energy absorption to prevent the screen cracking for low height drops.
Overall the Street Skin for iPhone 4 offers fairly good protection without adding huge amounts of bulk. Some might not like the tire tread patterning, or the rubber texture, but it's available in pink, white and black for both the CDMA and GSM variants of the iPhone 4 from US$29.95 and makes a decent everyday case.