The PR pieces I've seen have all used very tricky wording to make it seem like it's more successful than it really is.
It wouldn't surprise me at all if the PR is being "clever" with its data interpretation. It's like when Apple announced that its original iMac machines had outsold any other model of PC; well, there was only one model of the iMac, but the same Dell or Gateway may have been sold in dozens of configurations, therefore their sales were diluted against dozens of different "models" despite being the same machine.
"Lies, damned lies, and statistics" or something like that. Despite the clever phrasing, Apple did not continue on to dominate all PC sales, everywhere.

So the same could be true of iPhone in Japan, where the sales of the unified iPhone 3G/s since its launch has sold better than any other single model of phone from the other manufacturers. Doesn't mean that it's actually got a sizeable percentage of the market.
The larger issue with Apple isn't just the lack of pre-encoded emoticons, crappy camera, and (until recently) high price tag; let's not forget Softbank's ABYSMAL reputation in Japan despite two series of very clever TV ads. The general perception of Softbank is their customer service is bad, their signal strength is the worst among the major carriers, and their contracts are DRACONIAN.
A kiwi friend of mine went in to get the 3G/s and took his wife to help him understand some of the fine print in the contracts. If he loses his phone, they will charge him an additional 10,000 yen (
a little over US$100 at current exchange rates) per month on top of the base limitless packet charge of 7000 yen. This additional monthly charge runs through to the end of the 2 year contract, even if he loses the phone tomorrow. There is no explanation for this additional monthly charge, they just looked at him as though a bug had crawled up their but -- "Ooh, the foreigner is getting uppity."
If he wants to replace it through them, they'll charge him 80,000 yen for a new one, so he's thinking to buy a US unit for 300 gringo dollars and then having them produce a new SIM card for him. I don't think that will work.
Anyway, when he was done describing all the contractual ways they could rectally service him for two years, I was a lot happier that I've got an iPod Touch and a separate Casio cellphone which records movies, audio, still photos up to 5.1mp and has a decent optical zoom. It was free.