Kal-El (Tegra 3) will have double the cores (Quad core) with 20% higher clocks (1.2G) and a faster iGPU than Tegra 2.
Latest news put pads with Tegra 3 to launch in October
I dont expect Tegra 4 (Cortex-A15) before Q2 2012 and products could be late as of summer 2012.
The number of cores aren't terribly important for most applications, so that's largely irrelevant. A lot of the common workloads for tablets is offloaded to dedicated hardware on the SoC whenever possible as it's much more efficient than using the actual ARM cores. Switching to an A15 on a better process will lead to significant performance improvements. Supposedly A15 is upwards of 40% improvement, clock-for-clock. A 28nm process will allow it to be more power efficient as well.
Even though it may launch in October, it will probably be a long while before it's include in any flagship products. The Tegra 2 was available long before it actually made it into any tablets worth buying. The first products to actually ship with a Tegra 2 were terrible even by the standards of the time it launched.
I don't know when Cortex-A15 products will be available, but I'd bet money that Apple will use the newer design in their next SoC for their next iPad. Assuming no one else beats them to the punch and they keep their same schedule, they'll be launching early 2Q 2012.
The GPU improvements will be welcome, but outside of a few applications, they won't be terribly useful. I don't think the performance increase will be worth the wait, and with better chips in flagship tablets coming out in 2012, I think the Tegra 3 is in an uncomfortable middle ground.