MLC (http://searchsolidstatestorage.techtarget.com/definition/mul..., http://birchtree.me/blog/nand-flash-party/), which is used quite a bit in consumer flash, is base n>2.

Those links claim devices use n=4, 8, 16,... but apart from practicalities, I don't see a reason why n=3, 4, 5,... wouldn't work.

Also, these devices may get produced in power of two sizes units, but they all use error-correcting codes that turn their effective size in something non-power of two.