Ive seen people claim that all artifical forms of B12 cause mercury methylation.
That being said, I have not seen proof that methylcobalamin, for example, is actually a methyl donor to create methyl mercury. Supposedly, according to b12d.org, methyl mercury can only be created by artificla means and not within living organisms.
A little excerpt from b12d.org.....
"Cobalamin (the bit that hasn't got methyl on it - also known as B12) spends its time floating around looking for heavy metals. It binds to the heavy metals, and the kidney looks out for cobalamin bound to heavy metals (mercury is a heavy metal) and flushes them out in the urine. Actually cobalamin looks for most toxins and flushes them out of the body - that's one reason why alcohol and smoking both reduce your amount of B12 (and cause all sorts of nasty diseases) - because the B12 is doing its best to remove the toxins and is being removed at the same time.
So if you are worried about mercury or any other poisoning, then take B12 to remove the poison. It certainly won't make it worse and most likely will make it a lot better"