[hpbaxxter]

Edit: I can boot Beta 5 all the way up to sdram_freq=499, but as soon as I try to use sdram_freq=500 the Pi will fail to boot (2-3 flashes of green LED, then nothing).

This same Pi has been 100% stable with sdram_freq=600 up to and including Beta 3 - something iffy in the post Beta 3 firmware/kernel?

I had to insert boot_delay=1 in config.txt to boot with higher sdram frequencies than 425 MHz; 500 MHz now works fine. Maybe it works for you, too? I haven't checked if it's still necessary for me with newer versions as this behavior already appeared in summer. For raspbian, however, this parameter isn't necessary at all, so this seems to be a bit kernel specific.