I am hoping this is a bug in CS5.5 because I have done this successfully many times in CS5.0. The wide screen menus I create all have the 16:9 aspect ratio checked but on my television they are coming out 4:3. This is on a just burned bluray from the CS5.1 Encore. I can't find any project settings that would have done this, but under the menu viewer there is a new button called 'correct menu pixels for tv display' ; when you press this button the wide screen menus scrintch-up like 4:3s. I never noticed this in CS5.0, but they should render like 4:3s if they are widescreen if you have the correct aspect ratio checked.
What's the size of your menu? 720x480?. Always use HD size (1920x1080) menus for blu-ray. If I remember correctly, "Correct Menu Pixels for TV display" was introduced in Encore 2.0. It's only for preview purpose and won't affect the output. If you use any menu with square pixels then you don't need the option, this is useful when you are working with non-square pixels.