I listen to Pandora Internet Radio quite a bit and I hated the ad shown above the player ... until I found Stylish could hide it! This very short CSS addition hides that all-to-familiar box.

Also eliminated the left-margin on the player, the top button row and the Pandora Tips box, allowing the window to be even smaller! The site's CSS code eliminates scrollbars for the page, so even if you want to resize the window a few pixels smaller than the player is big, those pesky scrollbars stay out of sight.

Note: Screenshot shows top row buttons and tip box still visible.

(sry, terrible screenshot)