OFO,
If I recall correctly, the US has leant heavily on Japan post '9/11' to reform their constitution, especially with respect to the deployment of the JSDF, hence why they had troops in Iraq and Afghanistan (albeit in 'reconstruction' roles). Furthermore, the US 'assisted' the JSDF to become much more multilateral in the maritime security field by getting their ships and MPA to participate in high seas MIO against vessels beleived to be carrying WMD precurser material, under the legally dubious 'Proliferation Security Initiative'.
Other countries in the Pacific Rim still feel rather uncomfortable about the overt military growth of Japan, beyond what are clearly self-defence means. I also recall a Dan Pletch lecture at RUSI where he asserted that Japan could be the next nuclear power - it has all the pieces, including launch bodies and fissile material (the latter heavily controlled and not reprocessed in Japan...but guess where?) - and more importantly, it has the perceived defensive need - DPRK and China providing two prescient reasons. Obviously there is an enormous (and possibly insurmountable) historical legacy associated with nuclear weapons, but the strategic leverage that the US and Japan could assert on China if Japan had an 'independent' capability would be enormous, akin to the Soviet Union placing weapons systems in Cuba...
...mind you, that didn't quite work out as intended!