I am with pdr here--do not include a statement that you are unpublished. The lack of credits in your cover letter will already tell the editor that, so stating it outright is redundant. And no matter what, even if a market is especially friendly to new writers, the writing itself must stand out. Your story will not be bought simply because you included a statement saying you were new.
Essentially there are three basic levels of cover letter:
1. Unpublished
Dear Awesome Editor's Name,
I have attached "My Cool Story", a 3400 word <genre> short, for your consideration. Thank you for your time.
Best wishes,
Me
2. Recently made a sale, but the sale has not yet been published
Dear Awesome Editor's Name,
I have attached "My Cool Story", a 3400 word <genre> short, for your consideration. My fiction is forthcoming in
Super Nifty Magazine. Thank you for your time.
Best wishes,
Me
3. Reputable publishing credits exist
Dear Awesome Editor's Name,
I have attached "My Cool Story", a 3400 word <genre> short, for your consideration. My fiction has been published in
Super Nifty Magazine,
This Other Super Nifty Magazine, and
Wow Another Nifty Magazine. Thank you for your time.
Best wishes,
Me
These are the basic templates which have served well me so far (with minor variations to the last one now since I have more than the three credits it is comfortable listing). A bit dry, but they have all the info an editor needs to know where you are at in your career.
-Michelle