No, this isn't one of those urban legend that I'm passing around in blog form.
My friends have a daughter who developed a rash last night. After going through a list of everything that they gave her, we narrowed it down to giving her some lemonade with Splenda in it.
A quick google search turned up lots of discussion about sucralose allergies. Splenda contains sucralose.
So, if you have an unexplained rash and you use Splenda, stay off of it for a while and see if that treats it. It can be treated with Zyrtec or an antihistamine such as Benadryl gel.
Of course you should consult your doctor or pharmacist before treating the problem.