I have a Toshiba 30HF66 that has 540p mode. It only works over component or HDMI and only when sending a 480p source signal (at least on this TV). No 480p source = 1080i even with 540p selected. I've found 1080i looks better and tend to use it more. I only use 540p (480p) when using my Wii console for gaming. 1080i looks better for HD video sources and upscaled DVD.
A friend had a 540p mode working with the output of an ATI card onto a 32 inch HD CRT Sony, there were no scan lines or jitter, it looked very nice. I don't have the details because I don't have contact with that person anymore but they used it as a computer monitor at the time.