Not yet- here in No. Ohio's snow belt we usually have snow by this time, but not this year.
Reasons why the '05 might just be OK in snow:
1. Ford says they did over 1 million miles in snow- No. Michigan and in Sweden during the years they spent developing the '05 Mustang. Snow performance was one of their objectives as part of making the car more of a "daily driver".
2. "Smart" traction control- the TC in the 05 (I don't know about previous) senses the amount of slippage (I think on a constant basis) in the front wheels, and then adjusts the TC to the conditions. Lots of control when the pavement is slippery- and actually allows some slippage when the traction is good.
3. All-weather tires- the Pirellis (GT) are rated as M&S tires (also 400 as to wear)- so they are right in there with the best all-season tires. They are definately NOT "performance" tires that have 5/32 tread depth- they are full-depth and designed for rain and (light) snow.
I think they just made the '05 a better all-around driver than some of the previous models- ride quality, room, trunk, noise, etc..
All this said- you KNOW that the Mustang (or any car) will not outperform a truck in deep snow, right? It's still a car.