If you don't have the flexibility of a hydroelectric system...well, here's some evidence from Europe, from Motl:
Every year, a huge excess of wind-generated electricity from Northern Germany causes problems to the grids in Czechia, Poland, Austria, Slovenia, the Netherlands, Belgium, and Switzerland.A commenter at Motl's blog notes that coal and oil thermal plants that have to stand down because of wind have to be put into "hot standby": they're still burning fuel to keep the plant's temperature up, but they're not generating electricity.
A year ago, the wind was really strong and the first problems occurred. So they established a warning system. The problems are repeating in 2009, too. For example, it was planned for the last week that there would be a total of 130 MW of electricity flowing from Czechia to Germany. However, the actual budget was 1300 MW in the opposite direction. The balance was almost 1500 MW different than planned.
Mr Petr Zeman, the CEO of the Czech Power Grid (ČEPS), explained that they have survived so far. In his opinion, the problems are caused by the uncoordinated development of the wind turbines in Germany. They have stated that 20% of the electricity should come from renewable sources but these slovens no longer care what it does with the grids.
It is clear to Mr Zeman that if Germany were not connected to the rest of Europe, it wouldn't be able to survive these moments. The grids could be built to sustain such irregularities. However, it takes something like 10 years to build new power lines (which includes 7 years of the hugely complex EU paperwork).
The blackouts are being avoided by rather ad hoc methods of turning individual stations on and off in an internationally semi-coordinated and semi-predicted fashion. However, the chaotic description makes it likely that the system may collapse at some moment. The tasks for the regulation will be increasingly difficult as Germany wants to inflate the current 25 GW of pinwheels to 50 GW of pinwheels.
Sounds like a great deal: pay for wind turbines, keep paying for fuel for the oil and coal plants, and get the added bonus of grid instability.