[siliconviking]

Could this result in ETH splitting up into two currencies again, like it did last year? If so, there will be tax consequences / realized gains for those who sell out of the version they don't want to hold... further complicating things.

[SimonPStevens]

Almost certainly not. The expected outcome is that everyone will update their software, and the old fork will die.

Unlike last year this is not a controversial fork. It's just a planned part of the network upgrade process. No groups are planning to boycott the fork.

(It's futher enforced by code in the old fork that will cause it to die shortly, so even if a group of people wanted to keep running the old fork, they would actually have to hard fork it themselves to remove the 'ice age' anyway. So it would have to be a concious act, rather than just people forgetting to upgrade)