If you know it's going to happen and you have a decent grid you can manage. The UK supposedly has a 3GW spike in power draw every time the BBC's soap opera goes to comercial. That's a sizable chunk of total power in an island grid that doesn't have the rest of Europe backing it up.
http://www.geek.com/news/tea-time-in-britain-causes-predicta...
You're right that the increasing number of grid-connected devices could lead to problems like this, especially as more and more appliance makers start to add this ability. At the moment I've seen grid-enabled devices used to back down on electricity use during peak times. ComEd offers a GSM-connected box to modulate your A/C compressor during brownout times.
But I've tinkered with a thermostat that superchills my house when the price dives into low or negative space. I guess I never thought about what could happen when 1,000,000 homes do the same thing. Law of unintended consequences I guess.
You're still not paying the real-time prices of electricity, for which the purpose is to balance supply and demand.
The pricing systems seem to be designed to be easily understood rather than completely practical.
As a result, the price may jump every day at 5PM, and then go back down at 10PM. But I fear we'll eventually see a time where HVAC/dryers/fridges/etc turn on full blast at 4:30PM and stop entirely at 5PM, then turn on again right at 10PM, leading to grid instability due to the sudden change in usage at exactly one point in time.