Never been, but I would recommend using the search feature here, entering "food and Wine". I remember seeing lots of threads. This is in case you don't get any responses.
If the Signature Dinners are like the Party of the Senses, then no shorts, t shirts, tennis shoes. There were a few other things too. If you call the Dining number (1-407-WDW-DINE), they will give you the list.
FMW
I have not been to a signature dinner, but we went to an Exquisite dinner last year. People all came dressed in dressy cocktail attire. I wore a very dressy pantsuit and was the only female in pants. Most women had on short or street length cocktail dresses. All the men had on suits or at least sports coat and tie. It was a fantastic experience with wonderful food and exquisite wines. The tables setting were very elegant. I am sure you will have a wonderful time.
We are looking forward to our F&W experience this year.
You can wear nice shorts to the Party for the Senses. The dress code is Disney's definition of business casual (same as code for Flying Fish, Cali Grill etc.)
When I made reservations for Jiko and the Cali. Grill about ten days ago, the CM said that the dress code was business casual - no tank tops or beach attire.
For signature restaurants shorts are fine for either gender. Tank top are not okay for men but sleeveless is okay for women. Sandals or sneakers are also fine, just no flipflops. Basically the idea is not to wear beach gear, ripped clothes or t-shirts (however some have not had a problem with Ts). I would assume it is something similar for F&W events as well.
The signature dinner we were at last year..men wore jackets/ladies nice dress or pant suit. Everyone was dressed quite nicely, no one in shorts/park attire type clothing.
I am wearing a short cocktail dress or dressy pants/top to the Spirited Ball. I am also wearing dress pants/top or a dressy skirt/top to Party For The Senses. We did PFS last year and saw maybe 2 people in shorts. I felt underdressed in a casual skirt and t-shirt. Most were in khaki pants, button downs, dresses, nice skirts and tops.
I think there is some confusion between the signature dinners with the food plan and the Signature Dinner during the Food and Wine Festival.
I've never seen shorts at the F&W Signature Dinners.
On the other hand, I've only seen a few people with jackets at a F&W Signature dinner. Webray - was that an Exquisite Dinner? For those, jackets are requiired.