There are not many things I don't like about Japan, but one of them is the difficulty that foreigners have to set up a SIM card, unlike most other places I've been to, I can't just go to the local 7-11 and pick up a pre-paid card.

While there are travel companies that will rent you a phone for your stay, a quick an easy alternative is to buy a Telstra pre-paid here in Australia before you go. Normally Telstra are the most expensive option, but in this case these SIM cards come with international roaming already setup and ready to go as soon as they are activated, and for basic SMS's, etc. the price is pretty affordable.

I bought a $30 prepaid Telstra SIM card from Kmart, activated online within about 15 minutes (although it can take up to 4 hours) and I was ready to go. While we are in Japan, SMS's between us will cost 75c and calls will cost $1.48/minute.
Also, if we happen to use more than $30 in our short 2 week trip, we can top them up online just like we were in Australia.
One more thing to note, is that most phones come locked to their provider, so if you're with Optus, etc. you might want to either look at getting your phone unlocked or some alternatives.

For more details on pricing, check out this page
http://www.telstra.com.au/mobile-phones/international-roaming/pricing-compatibility/