There is .flv support in the latest beta for the Live. And there will never be rmvb support because the chip in the Live cannot decode rmvb. The WDTV Mini has it because it uses a completely different chip.
Edit: Opps, noticed you do not have the Live version. Doesn’t look like WD is gonna come out with any new firmware for the normal HD version anytime soon, I guess you’re out of luckl. Sell it and get another player.