The next generation "E" (extreme) Intel SSD will be based on MLC NAND flash, like the "M" versions are. Benchmarks also show the X25-M G2 to be faster in some benchmarks than X25-E, so Intel proven that MLC NAND flash can be used in high-performance SSDs as well.
Of course, SLC provides other benefits like longer lifetime in write-heavy situations. Except for extreme scenario's, at this time there is not much reason to buy X25-E now that X25-M G2 is out. Next generation Intel SSD ("Lyndonville") will take advantage of SATA 6Gbps and command buffering.
What the Extreme series stand out for, is their lower capacity due to alot of the raw capacity being reserved to speed up random writes. The M series has less space reserved so its a trade-off between capacity/price-per-GB and performance. But it appears even the E series will be using MLC NAND flash memory when the next wave of Intel SSD's present itself.