i agree, that's a huge shortcoming, and i hope sony will get around to adding integrated dictionary lookup (and, you know, some dictionaries
) soon. while we're waiting, our very own nrapallo has slaved over a brilliant epub version of the webster's 1913 dictionary
which works a treat (i've got it myself). and it's free.
i'm working on an epub french dictionary but that might not be what you're looking for.