Get Complete Package.xmlJune 24, 2015
There is no way to figure out wild card metadata components (*) using API. Also, there is no way to figure out what components are not supported as part of force.com migration tool.
Out of above posts, I found Jitendra's post very interesting and useful. Whereas packagebuilder by Tquila guys, generates an exhaustive list of package.xml file for your organization.
On a side note Tquila guys have build a few interesting heroku apps.
https://sfswitch.herokuapp.com/ - To switch components between orgs.
sforgcompare.herokuapp.com - To compare metadata between orgs.
Watch this space for more info!