It turned out that the unlimited part doesn't really refer to the download speeds, since the plaintiff received a message that he has reached 10GB of data usage, therefore his access will be restricted to 50kbps until the end of the billing cycle. Calls to customer service resulted in refusal to lift those speed limits.
The lawsuit is similar to what Verizon settled in 2007 after advertising unlimited data plans, but actually throttling speeds down once an invisible cap was reached, and this is a well-known practice of many carriers.
Now, we know these class-action lawsuits are usually a dime a dozen in the corporate world, and the companies' lawyers usually quickly dismiss the charges. Moreover, there has been fine print at the end of the T-Mobile brochure, which states that the carrier reserves the right to restrict download speeds if the network is overloaded, etc.
What we can hope for, is that such lawsuits result in these warnings being made more prominent, with actual examples explained, and the data caps clearly stated, so users know what they are dealing with upfront.
source: ConsumerAffairs via Electronista