The name/term "Mustang" is used loosely on these cars (almost like a nickname) as they have very little, if any Ford parts. They only look like a Mustang, just as a RUF looks like a Porsche.
http://www.ruf-automobile.de/content.php?seite=&rubrik=&id=&lang=en

See this clip – note what the narrator says later on in the clip about the car…
http://www.edmunds.com/insideline/do/Features/articleId=121424

And here – The car does not have any Ford emblems…
http://www.edmunds.com/insideline/do/MediaNav/articleId=121424/firstNav=Gallery/- photoId=41635

Recently, Saleen has been given recognition as being it's own full fledged car company – not a tuner company. Because of this, Saleen is subject to the exact same federal regulations as Ford, GM and so on. Saleen numbers each car individually before it’s release for sale.

They mention Mustang in the text, but the way they mention the car it does not seem that Mustang is part of it's name. In the next occurrences they just say "S281 Extreme".

Not to keep beating a dead horse, but in the link provided by Neptune on the Transformers Saleen S281, the Saleen website does indeed use the word “Mustang”.
http://www.imcdb.org/vehicle_128980-Saleen-S281-2007.html
http://www.saleen.com/transformers.htm
As does the dealer locator portion of the Saleen site: http://www.saleen.com/dealerLocator/DealerLocator.aspx

I would be curious to find out how the cars are registered, maybe from someone who owns one. N.A.D.A. (National Automobile Dealers Association) also lists them as “Saleen Mustang”
http://www.nadaguides.com/default.aspx?LI=1-22-1-5006-0-0-0&l=1&w=22&p=1&f=5012&m=1348&c=13&z=63010&da=-1&y=2006&da-1
eBay lists them as “Ford Mustang Saleen” and when you decode the Vehicle Identification Numbers (VIN) provided on the eBay listings, the results identify the vehicle as a Ford Mustang...
https://www.fleet.ford.com/maintenance/vin_tools/default.asp So unless Saleen generates a new VIN, I don’t think the Mustang basis can be ignored.

Further, some still have the Mustang grille emblems in place. I am of the opinion it would be best to list these with “Saleen” as the make and “Mustang” as a portion of the model name. I believe that if “Mustang” were somewhere in the name, it would go a long way towards preventing confusion and minimizing arguments.