Sony & Amazon would like you to believe they have an oligopoly, because they want people to forget that dedicated ebook readers have been available for a decade, and that they aren't the only two e-Ink readers on the market, either.
Right now, e-ink ebook readers in the US include:
Amazon's Kindle line (K2 & DX)
Sony's PRS line (505 & 700, which may be being phased out, and the 300 & 600 being released later this month--the 300 has a 5" screen; the 600 & 700 are touchscreens)
Aztak EZReader, and Pocket Pro (5" screen) coming out later this month
And less well known, some of which are harder to get--the iRex line, which are still available in some places but harder to find (and much more expensive because they include more features), Cybook Opus, Foxit eSlick, CoolER Reader.
With those, you've probably got an oligopoly--a handful of companies with similar devices that are trying to fill the same market niche, with minor differences in abilities. (Some differences are less minor than others, and some are crucial to some potential customers.)
Barriers for new companies include the very expensive e-ink technology, and advertising--catching the eye of a public that already does their book shopping at Amazon, already recognizes Sony as a maker of single-purpose entertainment electronics.
Non-eInk readers include the eBookwise, the Jetbook, and the Franklin eBook, and a few others. (Potentially including the entire mobile phone/PDA lineup; all of them read ebooks of some sort or another.)
If you include the mobile phones & PDAs, you've got something else, neither monopolistic competition nor oligopoly. They're not all competing for the same market niche; PDAs that read ebooks are competing for customers with ebook devices, but they're not offering the same services.
Ebooks is a whole different market, not tied to ebook readers except for a handful of ebook stores (including Amazon's). There are dozens, maybe hundreds, of ebook stores (see list at http://wiki.mobileread.com/wiki/E-book_stores ), with new ones popping up all the time. Those aren't limited the way device companies are.
They're competing for the same customers--people who buy books to read--but they're offering different products, different levels of perceived quality, different bargains, different conveniences. They are as similar and as different as restaurants.