Wikipedia articles often have handy links to primary sources, though. Generally, encyclopedias haven't been allowed as reference sources for student projects, and in that sense Wikipedia is no different. Tertiary references like this are a good place to start, and can be useful to answer a quick question, but students need to learn to look for primary sources.
That being said, I think the Wikipedia lookup is a great feature of the Kindle (even though I don't own one). The iLiad can provide it using add-on browsers via wireless, but only if you're near a wifi hotspot. I don't think any of the other dedicated readers come close.