'Japanese blog Macotakara reports (via MacPost) that a China Unicom executive apparently confirmed during a presentation at this week's Macworld Asia that the "iPhone 5" set to be announced next week will indeed support HSPA+ "4G" technology, offering a maximum theoretical data speed of 21 Mbps.'
- per 'Macrumors'
https://forums.macrumors.com/threads/1236215/

So what if you have a theoretical maximum speed of 21 Mbps?
If with AT&T, you may be checking emails real fast, but will not use that speed to any effect in downloading many movies. For instance, say you wanted to take advantage of iTunes to download the HD version of 'Pirates of the Caribbean: On Stranger Tides;' good thing it is released in October, because at 4.51 GB if you had downloaded two films of similar length in September, aside from any other data use, you might have already run afoul of AT&T in being in their 'top 5%.'
Many may ignore this in thinking, 'well, I'm not in the top 5% of anything,' but that would be a big mistake. Because by limiting the top AT&T is effectively saying
no one can use much more than 10 GB in a month, not at any reasonable speed that is.