jQuery plugin which will print to a printer an element or a jQueryUI Dialog
From a once over: 


I am not a native English speaker but //Element to print HTML confused me. Maybe //HTML elements to be printed?
delete settings; is meaningless in destroy ( see comment )
elems=$(settings.elems); should be var elems = $(settings.elems); otherwise you pollute the global namespace
You declare var i twice in a for loop
You should consider removing the iframe in destroy


All in all very clean, maintainable code.