Carl has a nice post up on chromosomal changes on the heels of an article he wrote on the topic recently. He references two papers, one comparing the macaque and human genomes, and another that compares various mammalian orders. I have previously pointed to a paper (you can read the full text at the link) which reviewed gene expression differences in human and chimpanzee brains and found that differentially expressed genes tended to be found on the 10 chromosomes which seemed to be rearranged in humans vis-a-vi chimpanzees. The correlation between these two genomic features (differences) is certainly worthy of further exploration. Also, please note that the domestic and wild (Przewalski) horse have 64 and 66 chromosomes each, and hybrids have 65, while another cross with the domestic horse results in 64. This page has more references of similar crosses (different chromosome numbers in the parental generation but viable fertile offspring).