Hmm. The strace rather suggests you messed with your config file in a nasty way.
open("/var/cache/apt/pkgcache.bin", O_RDWR|O_CREAT|O_TRUNC, 0666) = 5
lseek(5, 50000, SEEK_SET) = 50000
See, that number defults to a couple meg, not 50k.
Jason
