Online Attack
Having things such as increasingly powerful graphics processors you can run CUDA crunching on is all very well and good, but kind of irrelevant in the context of web based attacks.
Consider a password which may be between 1 and 6 characters long, alphanumerics, giving a total of around 2 billion options, lets take another mathematical shortcut and ignore the missing digits from the smaller numbers and lets say that each option tried is 6 digits... so for each check you've got 6 digits, lets add 250 bytes for a decent sized HTTP POST header and presume that you're also going to need to send a 10 character login name and, while were at it, the fields will need to be identified so 'user=' and 'password=' add another 14.
That brings it to about 270,000,000,000 bytes to transfer or about 250 GB of upload to the server.
Lets presume that in order to know if you've succeeded in logging in or not you're going to need to receive the response, and for the sake of argument lets say your average webpage being about 15k totalling an additional 28 TB of bandwidth.
So all told you're talking about 28 TB of bandwidth to check all of the 6 character passwords for one user.
Now the question is, if you maxed out the bandwidth of a moderately sized server of the kind you may wish to attack without alarm bells going off all over the place due to the expensive DDoS and IDS protection you find on larger sites.. so let's say that's 10 mbyte/sec... about 3 million seconds to test them all or 30 days.
Using the assumption that somebody wouldn't noticing you sucking up 100% of their bandwidth for an entire month you then have to consider the poor server trying to check all of these details - running a password attack on an offline is all very well and good... but what is a server going to think when it's having its CPU burnt up by handling billions of extra page generations in ASP or PHP or whatever it may be.
Anyway, in summary, it is true that longer passwords are needed... but when you're dealing with websites, how many you can shove down the pipe to be processed by the server is much more important than how you generate the passwords in the first place.