My Samsung PN50A450 3D Ready plasma is compatible with Checkerboard and Horizontal interleaved and Vertical interleaved. I've used the Checkerboard mode with my Xbox 360, and it works perfectly fine with James Cameron's Avatar the game. I have no idea what Horizontal interleaved and Vertical interleaved means.

Can anybody explain what this means?

It means how the rows/colums stereodata are aligned. In a horisontal interleaved image the odd horisontal lines contains the image for the left image and the even lines the image for the right eye (or vice versa...). With vertical interleaving the images are aligned in a similar way but in vertical lines instead. With checkerboard interleaved the images are aligned like in a checkerboard (white squares for right and black for left eye).

Does the fact that it can do horizontal or vertical interleaved help me in any way with trying to convert field sequential to it? Or for all intents and purposes is it basically a checkerboard 3D display, and I can just forget about the interleaved part of it.

There's really no use for hor. and ver. interleaved since checkerboard 3D looks bettar. Stereoscopic Player can play most 3D videos and display them in checkerboard 3D on Your TV.

I agree with Freke. It is possible to convert a framesequential movie to interleaved but different compression algorithms doesn't work good with interleaved images (basically because those algorithms mixes the data for left and right eye views resulting in a crappy experience).
For homeuse i recommend side-by-side since original resolution is kept and for movieviewing the stereoscopic player rocks! Always keep your original media and try to play them using the sterescopic player.
If you want to try making a video into interleaved format then i recommend stereomoviemaker.

Do the PN42A/B450 or PN50A/B450 TVs support side by side ?

Having an option for horizontal interleave helps compatibility. some software might have an interlaced mode but no checkerboard support.
even if the image quality is lower, at least you can use this program. (example : youtube 3D, which supports line interleave but not checkerboard)

No, A450/B450 series only support 60 Hz progressive signal in VESA modes.

Best way is try and find a stereo player that supports both interlaced stereo and checkerboard output, so all the necessary transcoding is done in the player.

No, they don't - checkerboard, horizontal interleaved, and vertical interleaved formats, VESA 1360x760 60Hz mode only.

C7000 series do seem to allow 3D in all supported VESA modes; all the above formats with the addition of side-by-side (half) and top-bottom (half) formats