I kind of hope this rumor is wrong actually. If Canon really intends to announce a 5DIII in March or April it's probably not going to be the camera many of us want. The 5DII severely undercut sales of the 1Ds III as the image quality of the 5DII was better, it had a larger ISO range and files were more flexible in editing. It's true that the 1Ds III had better weather sealing, a longer lasting shutter and dual card slots along with a far superior AF system but for a lot of people that wasn't worth triple the money.
If a 5DIII is announced in March or April it will not be a mini 1Dx as that would kill 1Dx initial sales. So an early announcement like the one rumored means that the new 5D will likely be the camera many of us hope it's not; a slow focusing, way too high (uselessly high) megapixel camera that doesn't compare to the 1Dx in high ISO performance so that people who were waiting for a body with the 1Dx's features won't consider a much cheaper option in the new 5D. A later announcement could mean a repeat of replacing the sensor with the flagship sensor after the flagship body has had a healthy amount of time on the market.
Alternatively the upgrades to the next 5D could focus on video. Either way, for still shooters I don't see an early announcement as a positive thing.
Hopefully they won't go overboard on fear of losing 1DX sales and have everyone just decide to keep their 5D2 or go D800 instead.
If it has 10-15MP increase and the rest is the same, not sure that would go over very well at all.
If the video is also much improved (no moire, etc.) that would be a big plus and that might bring some of the film guys on but some might wait for it in the 7D2 or 70D to get it at lower price or for when closer to 35mm film size is better.
If they also fix up the AF to a huge degree, maybe even go 1D4 AF and bump fps to match 50D then they really have something impressive though, IMO.
Hopefully they would consider weaker sealing and only 6fps and not the full on new 1DX AF alone enough to not worry about 1DX sales too much (the people who really must have 1DX speed won't want to settle for 6fps, no way, at least not for the main camera and they might want the, hopefully, ALL THAT new 1DX AF too, at least for the primary body and if they lose a few second body sales here and there to a 7D2 or 5D3 then so be it better that than to Nikon or to people just sticking with whatever they already have? and those who will compromise a bit will compromise anyway once things are just a bit off the top so why crippled the lower lines THAT much).
Part of me actually thinks they will do it right and the summer release also seems better than the winter releases too (people can get it in time for big summer trips, summer photography, weddings, summer and fall sports hmm maybe they want both bodies out for London 2012 too as opposed to the ever later in the year releases when the bodies would come out too late even for fall foliage or end of fall wedding and sports seasons. Yeah, yeah, that is being a bit northern hemisphere and cold weather latitude biased but that is where probably a large chunk of the world's camera buying population lives).
But part of me thinks they will be same old slow, reactive kings on the hill (until they one day fall) dribbling out scraps, as few as they possibly imagine to dare after spying on Nikon, and it will be 30-36MP, 3-4fps, slow response, long mirror black out, a crippled (or by a miracle full) 7D AF system only (and I find that for some things I actually trust 7D AF less than 5D2 AF I should say). They don't seem to be in the mood to push things and be charging forward on all fronts like the old Canon. Personally I could easily see just grabbing a 70D or 7D2 for better video then and sticking with 5D2 or maybe finally giving Nikon a good look if the D800 is the monster that many rumors claim if they hold the specs back too too much.