I did a little bit of research, but found no link between rainbows and snakes in Japanese mythology. I did, however, find some neat parallels between the Naruto universe and mythology. Orochi was a giant 8-headed serpent who was killed by a god named Susanoo. Orochi demanded virgin sacrifices, and was a cancer to the region in which he lived. While cutting Orochi's tails off, Susanoo found the sword of Kusanagi.

If you look back to episode 71, you see that Orochimaru uses the sword of Kusanagi against Sandaime. Orochimaru demands virgin sacrifices too.

These are just a few parallels, but maybe there's some foreshadowing here. In the mythology, Susanoo is the brother of Amaterasu and Tsukuyomi. If we look back to episode c. 95, Uchiha Itachi uses techniques called Amaterasu and Tsukuyomi. Maybe there's a Magnekyo Sharingan technique called Susanoo, and maybe Uchiha Itachi will use it to kill Orochimaru. After all, we already know that Orochimaru is afraid of Itachi. There's an even more interesting possibility. Itachi told Sasuke that there was another living Magnekyo Sharingan user. Maybe this mystery person will kill Orochimaru. (OK, now you can accuse me of reading into the anime too much. ;-))

Oh man, all this speculation is getting my psyched for the new non-filler episodes!