I wrote a novel, and then deleted the whole thing. Here's the take from what I see.
Japanese like the pacifist thing - they don't want to be in Iraq. Koizumi made the decision to send the SDF there without Diet consultation, and changing the constitution can't be done without public consultation (a long process).
No SDF in Iraq
Where Japan wants to exert itself is in protecting it's territories, especially the rocks in the ocean in the south that are claimed by every country in the region. A recent submarine fray has people really p*ssed. The biggest concern here is North Korea and the sheer size of China.
Chinese sub in Okinawa
The relationship with the US forces is fractuous among the people who live in Japan - the government needs it and has a great relationship with the US (they need it). But locals don't like living near a
military base. This was highlighted recently when a helicopter crashed on a university campus in Naha, and the US military refused to let the Japanese at it.
What to make of it? They won't be a power soon, and because Koizumi's government got almost throttled out of power in the last election, (they are no longer a majority ruling government ) it won't happen soon. I could see them offering up a patchwork solution to the American military presence in the short-term, but it will be hard for them to assemble anything resembling a true superpower force.