Have you noticed any tolerance or know of any studies that cover this?

I've seen neither. I take pretty much the same dose I did way back in my early teens when I started. But I'll admit, I haven't looked very hard for the latter.

A quick Google Scholar search only turns up a few hits for 'melatonin tolerance' which cover tolerance of melatonin itself; one says

Sleep maintenance and initiation were further improved following the 2-month 1-mg sustained-release melatonin treatment, indicating that tolerance had not developed. After cessation of treatment, sleep quality deteriorated. Our findings suggest that for melatonin-deficient elderly insomniacs, melatonin replacement therapy may be beneficial in the initiation and maintenance of sleep. http://www.ncbi.nlm.nih.gov/pubmed/8552931

Or in one primate study:

Daily administration of a 5-μg/kg dose for 4 weeks or gradually escalating melatonin doses (5-320 μg/kg over a 3-week period) did not result in the development of tolerance or sensitization to the effect of melatonin on sleep initiation or sleep period. http://cat.inist.fr/?aModele=afficheN&cpsidt=13688537

And here's some Times coverage:

On the other hand, says Wyatt, people don't seem to build up a tolerance to melatonin, and even small amounts, like the 0.3-mg dose used in the study, are effective. Studies show also that melatonin is safe for adults, at least in the short term, with few side effects. http://www.time.com/time/magazine/article/0,9171,1191851,00.html

But on the other hand, a regular Google search will turn up several people complaining of tolerance. (Whether you believe any of them or write them off as freaks, exaggerators, victims of fake melatonin, or whatever, is up to you.)