Every 3 days (on average) a block will take more than an hour to solve. Although 10 minutes is the average, occasionally it could be much more than this.
An hour is a long time to wait for a single confirmation, especially if Bitcoin becomes widely adopted.
It might not happen very often but this doesn't mean it's acceptable - ( once every 3 years a block will takes more than 2 hours to solve! )
I guess it's hard to propose a solution to this but could the difficulty be reduced (temporarily) if a block has not been solved in say, 20 minutes?
I realize that time is a fuzzy concept in the Bitcoin network so this might fall down if the nodes can't agree on what actually is 20 minutes.
This is just a natural result of probabilities. Sometimes you will need to wait a few hours for a confirmation, and sometimes you will wait a few seconds. As block confirmation times get further away from the 10-minute average (in either direction) the less probable it is. Ten minutes is just the expected time to solve a block based upon difficulty as it relates to network hashrate, but there will always be some variance because solving blocks is a random process. If you want, you can transact in a currency with a shorter average confirmation time. Litecoin's average block confirmation time is 2.5 minutes, but you'll still find occurrences where a litecoin block takes a half-hour or more. It would be about as likely for a litecoin block to be solved in 30 minutes as it would a bitcoin block in two hours.
There are also coins which retarget difficulty much more quickly, but this tends towards network instability (i.e. the difficulty varies greatly, and accordingly so do the number of miners).