I currently read exclusively on a Kindle, and have become totally addicted to the luxury of having the dictionary available at the bottom of the page, and the ability to quickly look up words on the fly.

I want to buy an ereader that supports DRM-ed ePub books. Which, if any, of the currently available ereaders supporting ePub formatted ebooks can provide an identical, or very similar, dictionary interface like the Kindle?

Bookeen's products have a dictionary in their Mobi firmware, but not in ePub one.

Look at Onyx Boox 60/Bebook Neo - the dictionary is buggy but very usable. I read many books with it.

Bugs:
- when changing the page you have to close the dictionary and open it again on the new page (if still needed) or use stylus to change the page,
- there is no option to enter a word to look up right now (will be in future firmware probably because there is place for it on the dialog),
- dictionary window is taking some place on screen, to move it to top/bottom you have to select a word or words close to the border of the dictionary window.

Also the dictionary is stardict - you need to download dictionary files for it (best English-English is Wordnet but there are many more).

The SONY PRS-600 has it too.
Irex DR800/1000 was supposed to have it too but I believe they removed it in the latest firmware because their dictionary lookup was relying on mobipocket support :(

The slickest dictionary interface is probably that of the Sony PRS-600/900. The Pocketbook and Neo/Onyx devices have a dictionary interface which, although less "slick", certainly works well enough.