Well, nothing out there consumer broadband that I'm familiar with is truly unlimited in the first place. They usually have 3 characteristics that are limited: time available for use, speed available for use, amount of data available to download.
In this case, 1 of those items is truly unlimited while the other 2 are not. You have an unlimited amount of data available to download during your plan. The speed and the length of time that data is available is up to the company. An alternative mentioned previously is the ipad plan. It is not unlimited (nor called that) because they changed the way it worked soon after release. It is limited in all 3 senses: their speed, maximum amount of time you can use it, and the amount of data is limited because when you use it up they either cut you off or you buy another plan. Compared to that, this plan has an unlimited characteristic. They do not cut you off early, no matter how much you use it or how often you use it. They merely adjust the speed at which you can use it.
Realistically, I guess none of these are unlimited. I have multiple internet connections, all at 15+mbps down and load balanced. I would consider anything less to be limited.
The real question is what criteria do you use to determine if something is unlimited? I don't believe anyone uses speed as a criteria nowadays as that's a variable spot to hit. B2B's normal 3mbps not unlimited but the slowdown of the new ToS is? Nah, 3mbps is pretty limited when you're used to faster. Obviously they do not cut you off so the access is not limited, whereas others cut you off completely. The time is obviously limited as it is in most cases where you pay for broadband - the lifetime of the plan, at which point you have to pay again.
So, how does this change the service from unlimited to limited when it was previously time limited, you do not get cut off early for downloading x amount of data, and only the speed has changed (but still something above 0)?
SPECIFICALLY, this question is directed towards those crying foul about it changing and 'no longer being unlimited'. NOT towards those that do not feel it should have been called unlimited in the first place. This is to cut off people arguing the other question about whether ANY broadband service should be called unlimited so long as it has any (time, speed, amount available to download) limits. I really hope there are mod points available for people that skim this response and reply that it should not have been limited in the first place.
So, why does changing the speed, not changing the length of time of the plan, and not removing the ability to access the internet at all, change it in some folks eyes from unlimited to limited - people that thought it was ok to previously call it unlimited and not now?
Several people mentioned things like downloading linux distros, or SDKs. Yeah, I download both, and more, and I can't even imagine doing those (simultaneously as I frequently do) on a 3mbps connection, so that really becomes relative to what the user is used to. Some people still download stuff like Runes of Magic (many, many GBs of data) on 128kbps connections and are perfectly fine with it. You, from the release, may still be able to do these things, just at a slower speed than you were used to.
I may have read the post wrong. It just sounded like it was ok to call it unlimited previously, but now it is not. To be clear, it ALWAYS had caps. The only cap that changed was speed, and that change is triggered based upon how much is downloaded.
LAST SECOND EDIT:
As someone else mentioned, this is closer to a true "Unlimited Data Plan" than most others. Many others actually will cut you off cold-turkey. This one still gives you access to the data, but alters the already existing speed cap to a slower rate. The time cap is not altered at all (unlike other plans) and it remains the same - however long the plan was to begin with. No new caps are introduced. The speed cap was altered. Unlike other companies the time cap is not altered and the data access is not cut off.