We're told Cowell's baby mama Lauren Silverman set up everything for the celebration dinner at Sur Lounge in West Hollywood ... where celeb guests included Ryan, designer Rachel Roy, and Lisa Vanderpump. Makes sense, since it's her restaurant -- plus, we're told LV and Silverman are longtime pals.
Seacrest surprised Cowell with a mini Maybach. The ex "American Idol" co-stars have a running joke about Cowell's 5'9" height ... which is funny, coming from Seacrest.
Based on Simon's reaction, size doesn't matter to him. Not on his birthday at least.