"I say toh-mah-toh, you say toh-may-toh, you say poh-tah-to, I say poh-tay-toh..."
There may be more to the song, but I never heard it. I remember my Gran repeating this rhyme to me when I was around seven years old, when my sister and I were visiting her flat in Glasgow. Actually, up until now (when I had thought about it) I had had the idea that the song was about the differences between the US-ian and the British way of speaking. I have no idea whether or not someone with a US accent would pronounce 'tomato' and 'potato' as in the song.
Deeahblita informs me that the lyrics are actually from an old Gershwin song, Let's Call The Whole Thing Off, sung by Ella Fitzgerald/Louis Armstrong and Fred Astaire/Ginger Rogers.