This usage surprised me greatly the first time I saw it. I've seen in a few times since, mostly in archaic concepts, for example in this valediction: "Remember me, who am your faithful and obedient servant..." I guess "who is" would be the modern usage, but I'm pressed to find out why. Somehow everything's moved to the 3rd. sg. when the 'who' is thrown in.
Edit: looking at the examples page there seems distinctly to be "I who am" and "me who am". The former doesn't sound at all odd to my ear, whereas the latter is quite strange.
And yet, 'who am' when speaking in first person would be grammatically correct, wouldn't it?
I, who am also interested about its modern usage, was unable to find anything that would shed light upon the matter. Perhaps when people contracted it, it became who'm, which sounded too similar to whom and was confusing. But that hardly seems an excuse to defy the rules of grammar.