Amazon probably has ~5hrs/year of complete failure of a region. Figure, conservatively, it would take 3 months of engineering time to protect against that, plus a 'continuing' cost of 1/2 a week per month to maintain that protection. You'd also have to (at least) double your provisioned capacity (which may include a larger ops team, etc). Assuming your servers cost $20k/month and devs cost $100/hr (both fully loaded), we're talking about ~$340,000 to prevent 5 hours of downtime (just for the first year).
If downtime costs you more than $50K/hr, then it might make sense to be that fault tolerant. Otherwise, there might be better places for a startup to spend its (limited) resources.
Instead, to go with amazon you have to architect for Amazon, not counting on your ECC instances to be up all the time, accounting for their local fast storage going away, or accounting for how EBS, which is persistent, is slow.
The alternative is, get the enterprise version of Riak, purchase dedicated nodes in two data centers, tell them about each other. (no engineering required.)
If engineering resources are the most precious commodity, it seems AWS is the more expensive option.
It's not unsolvable, but as others have mentioned it's probably not worth the time or money needed to have a live standby. There will be a level of failover speed that is worth having, but that might be "if EC2 is gone we can recover in 48 hours with no more than 6 hours of data missing" so the contingency will not kick in for a short EC2 outage.
