I remember seeing the 1998 Superbowl American Express commercial featuring an idea by Jerry Seinfeld that teamed him with Superman.
AmEx Commercial (1998)
According to these two teasers, they had gotten together again!
I had no idea that the two teamed up again in 2004 for new commercials. These are excellent, playing at 5 minutes long, these commercials show the relationship between Seinfeld and Superman if they were actual buddies.
A Uniform Used to Mean Something
Hindsight is 20/20
They even had an interview on The Today Show to promote the new commercials!
Today Show Interview
Here's an article by Animation Magazine about the 2004 team-up,