The Sampson Stone from Llantwit Major (Showing the Name of St. Illtud on the Upper Left Panel)
Although I now have several good arguments for Illtud = Uther (which, when taken together, make for a pretty unassailable case), my readers have pointed out what they believe to be two crucial flaws in my reasoning. I will address those in this blog post.
First, why use the title Uther Pendragon at all? Why not simply come out and say that Illtud was Arthur's father?
Prolonged use of a Welsh title derived from the Latin may have led to the personal name to which it was originally attached being forgotten. By the time of Geoffrey of Monmouth, Uther had become an independent entity, effectively divorced from Illtud. This process is nowhere better demonstrated than in Geoffrey's wholly fictional story of Uther, Gorlois (= gorlassar, a descriptor applied to Uther himself in the MARWNAT UTHER BEN poem) and Eigr/Ygerna at Tintagel. Such a process of differentiation between the epithet and the man who bore is unlikely to have been sudden invention. Indeed, this may have taken many years. Once complete, however, we had two entirely different characters. One was the father of Arthur, while the other - eventually transformed into a famous saint - was made a cousin of the great hero.
Second, even if Uther is Illtud, how do we know this man was actually Arthur's father?
Well, we don't. Plain and simple. The only thing we can say is this: nowhere in any of our extant sources is the earliest, most famous Arthur given any other father. As a consequence, we are not justified in selecting another paternal figure for Arthur. To the best of our knowledge - incomplete though it be - Uther Pendragon/Illtud was Arthur's father.
Of course, it goes without saying that if Uther were only made Arthur's father after the title became a self-functioning personage totally separate from Illtud, then we must question the validity of this genealogical link. Hero-making is messy business. It could well be that the title Uther Pendragon was chosen to stand in for Arthur's father because a) his real father was not known and b) U.P. had a nice ring to it.
Which possibility we decide to believe in is, ultimately, a matter of personal choice.