I'm not sure if this has been brought up before or not, but I was watching The Mambo Kings recently and I had completely forgotten about the whole I Love Lucy segment from the movie. I just wanted to see how everybody felt about that...I thought it was pretty interesting to see Desi Jr. play his father. Here's some information I found about it on the internet...

from Wikipedia:
The film features a scene where the brothers appear in an I Love Lucy episode, cleverly intercutting footage of Banderas and Assante into an actual I Love Lucy episode (where Assante and Banderas' characters appear to actually be intereacting in a scene with Lucille Ball). Desi Arnaz is played by his own real-life son, Desi Arnaz, Jr.

from IMDb:
Factual errors: The "I Love Lucy" show was filmed in Hollywood several weeks in advance of the air date and never broadcast live as portrayed in the movie.

Factual errors: The "Valentine" opening credits seen in syndication were *not* the original opening credits. When the series originally aired on CBS, the credits featured animated stick figures of Lucy and Desi along with the sponsor's product - Phillip Morris cigarettes, for instance. The "Valentine" credits were added when CBS began rerunning the series in 1958.