As a Welsh speaker with some knowledge of Irish, I can assure everyone that the word CAM definitely means CROOKED in both languages. It is a common word, in use today. In both languages the word has a secondary meaning of wrong/bad. Incidentally, in spite of the fact that Welsh and Irish are both Celtic languages, they are surprisingly different from one another, and it is uncommon to find a word which is the same in both languages. DU (black) is another, incidentally.
The BAN part of the word is more obscure. The common word for WHITE in Irish is BÁN, but somehow the idea of his being known as "crooked (and) white" seems far-fetched to me.
I suppose one possibility might be to leave behind the idea of CAM referring to the shape of the man's back, which on the face of it would seem the most obvious explanation, and think instead about the kind of settlement he erected. This might shed light on the BAN element. Maybe he wasn't a good fence builder! I say this because Irish has a now rather obscure word, BÁBHÚN (I do not think that the bh would not be pronounced here, or if pronounced would sound like a weak W), which has come into the English language as BAWN. Whereas the Irish denotes a walled enclosure, the English quickly began to denote the land enclosed (a very common linguistic phenomenon). BÁBHÚN also has an alternative form, BÁN. The BAN in the place-name BANGOR (my home town) may also be related with this BÁBHÚN/BÁN, incidentally.
Here is a link for BANGOR, which also mentions the question of the origin of the name.
http://www.tourwales.com/towns_and_villages/bangor.htm
Well, I hope this sheds some light. I think we can at least be pretty sure that the CAM part means crooked, whether this refers to his back or his fence!
Cheers,
Simon
PS Celtic adjectives have an extremely strong tendency to follow the noun, but there are nevertheless plenty of compounds made up of adjective plus noun. One off the top of my head is GLASWELLT: glas = green/blue and (G)WELLT = GRASS/STRAW. GLASWELLT = GREEN GRASS. A little tidbit I can't resist: Cinderella's "glass" slipper was probably a "glas (i.e. green/blue)" slipper!
PPS For those who are interested in Celtic languages for their own sake:
There are 6 Celtic languages, divided into 2 branches:
Q-CELTIC: Irish, Manx, Scottish Gaelic. The last two are descended from Irish.
P-CELTIC: Welsh, Cornish, Breton
The name comes from the fact that the first three tend to prefer a K/Q where the last three prefer a P.
PEN is Welsh for HEAD (in various senses). This becomes CEANN in Irish. The Irish used to have such a great preference for K/Q (the writing of this sound always involves the letter C in the Celtic languages) that they even changed the borrowed word PULSUS (originally meaning beating, but later referring specifically to the pulse in the human body) and changed it into CUISLE, changing the places of the L and S in the process. Many non-Irish speakers know the expression CUISLE MO CHROÍ, PULSE OF MY HEART, as a lovers' expression.
Notwithstanding, Modern Irish has a large number of words with the letter P.