First step in LTO-6 (native 3.2TB, 210MB/s) was made in June 2011 when the three technology providers announced the availability of licenses for this format. But later these specs were reduced to only native 2.5TB (+67% compared to LTO-5) and 160MB/s (a mere +15%). Generally, the capacity of the LTO tape follows current highest capacity of HDD. But it's now 4TB uncompressed and LTO-7 is supposed to be native 6.4TB (and 315MB/s) and will be there probably in at least two years.
So it looks like LTO-6 would backup 4TB uncompressed, pushing the original standard a little bit (no pun intended).