Once again, for the record, decreasing the time between blocks solves nothing. There's nothing magical about that sixth confirmation except that it took about an hour of processing time to reach. If blocks were found every 5 minutes instead of 10 we'd be waiting for 12 confirmations to get the same level of security.
There's definitely something magical about the first confirmation though; accepting coins after one confirmation is a lot riskier than accepting them after two no matter what the interval between blocks is, at least for reasonable intervals. Remember that malicious miners aren't the only threat; it's also possible to exploit natural block chain forks, and single-block orphan blocks are a lot more probable than double ones. (Also: if a big miner or pool redirects their mining power to attempt a double-spend, I
think it should be easier to detect statistically with 5-minute blocks and 12 confirmations required.)
Folks who were part of the ixcoin and i0coin launches should ESPECIALLY know this since the various exchanges and pools were all making you wait 50, 100 or at one point 1,000 blocks before coins could be considered "confirmed" because multiple blocks were being found each SECOND. It also illustrates nicely that if the amount of time between blocks were to be set too low, multiple miners would "find" a block at once and the network would argue over which was valid for a bit, which would create constant horizontal branching and orphaned transactions.
That would be an example of an unreasonable interval. Anything less than a minute is likely to cause breakage.