Thats a good idea, the GORM labs and when it comes to the name of the method I think it sounds easy enough.
On whether or not its necessary (although Robert don't want any input :P) I do think it would be good standard method for grails gorm, I am sure most folks have this code somewhere in their apps and should therefore be a standard one.
def item = Item.findByName('someName') ?: new Item(name:'default', property:'something').save(flush:true)
for example wouldn't work. It will return either true or false.
I hope you will provide two maps, one to find by, and one with the "extra" properties to create by and the flush option :)
Moe