Uhhh.... not quite.
The reason nobody can write code for the Itanium is because there are no good optimizing compilers out there that can do the neccesary code scheduling to take advantage of the VLIW architecture that the Itanium uses.
The 48 bit thing you are referring to I believe is actually 40 bit, and that refers to how much physical memory a processor can address. 2^64 can address an impossibly large amount of data. In fact, I read once that this can address more memory addresses than there are particles in the universe, or some such unbelievable number.
The 64-bit thing actually comes into play in that they actually CAN accept 64 bit instructions. That means MUCH larger data types (32 bit integers, etc) can be moved into the CPU much faster or in parallel.