Great and unexpected (by me) feature. It can and should still be improved in Marvin, and I have written about it
here.
By the way, Amazon has just announced this feature, or something very similar to it, for Kindle devices -- except that the feature also includes "flash cards" to test your knowledge of vocabulary items, or something like that. Well, flash cards can be useful, and the
display of dictionary definitions is superior in Kindle over all other e-reader apps (I love how those dictionary definitions pop up at the top or bottom of the Kindle screen, unobtrusively, with "Tap for more" links). At the same time, though, Kindle is extremely inflexible (as usual!) in terms of real-life use of this feature, because you can only use a
single dictionary at a time, while reading a book! And that's definitely not enough. For example, I typically use a
translation dictionary, in addition to an
explanatory dictionary, while reading a book -- but Kindle only allows you to use
one of them at any time.