Short "comment" version:
IMHO, 2006 or 2007 is more likey to be the year.
Long "essay" version:
I think we have _almost_ reached the point in the evolution of human-vs.-computer society that this may begin to happen. I don't think, though, that anything truely "mainstream" will happen this year although what proves later to be the first mainstream entry may begin its life in 2005.
Apple has succeded in making portable audio mainstream _almost_. It is mainstream within the limits of that portion of the population that are heavy computer users and has made usage practicle for those that are only, what I term, "software only skilled", that is those that are the equivalent of a driver that can drive a car well but have no idea how to fix one. They can fill the gas tank and load/unload the truck after a grocery trip but anything else requires a trip to the repairman.
eBooks become "mainstream" only when devices and services are on the market to allow similar non-techy usage. As long as the user must be aware of the aspects of differing file formats with any more detail than they now about different ice cream flavors (like chocolate/dislike strawberry = can use Mobipocket/can't use LIT ...) and need to own and use format converters eBooks will be outside of true mainstream society and religated to the computer nerd world.
Apple has proven that there is the possiblity that a publisher can make this happen. For audio, its taking (present tense because I don't feel the expansion into mainstream is quite done yet) a company in the position to be both publisher and hardware innovator. There isn't any company out there, or at least visible at present, that is in the position to do this with eBooks; a partnership will be necessary.
One problem hindering the successful move to mainstream is the extreme difficulty companies have in successfully balancing their craving to be everything to every customer (read: greed) and the limitations of current (including that seen in prototype) hardware. There aren't any displays, even publicly shown in prototype, that can do it all extremely well although a few are close.
The current expansion of the eAudio market (iPod, et. al.) into the eVideo market it in its very early stages. We are beginning to see some early precursors to what will probably be truely successful mass markets devices. The dispalys for these _DEMAND_ color and high speed refresh while not making much demand for resolution. Current PDA displays can match broadcast TV and sub-notebooks match digital HDTV with ease.
eBooks, on the other hand, want the characteristics of the Electronic Ink class of display. Slow refresh is fine, but extremely hirez and very low power demand are essential.
Those publishing media traditionally (print, DVD, CD, broadcast/cable, ...) make the most money from video and audio. Print is becoming an "almost ran". Any 2005 product that is significantly better for eBooks that what we had in 2004 will be limited to eBooks and audio books.
I think the big money, this year, will push toward apeing the iPod success with video products rather than eBooks though, personally, I long for a boom marked in ePublishing. I don't watch TV anymore (no broadcast TV out here I'm closer to Cuba than mainland USA, and I don't have cable) and I read 3-5 books for every movie (theater or DVD) that I see.
I'm happy enough with the state of eBooks as they are, but I have the skills to find and convert a more than adequate range of literature (there is no hope of living long enough to read all of the books at Project Gutenberg that I want to read much less any newer works). Prior to moving largely to eBooks, my reading was rarely things you could find more than one copy of at a Walden Books or Bookstop class of chain store if they had it at all. The current commercial eBook distribution rarely offers much that I want to read.