I suggest you create a smart list that looks for the following:

dueBefore:today

Then, when you go to that list, you can easily select all and postpone.

[That assumes you have overdue items spread across various lists; if they are all on one list, you can just click the select overdue link up top, and then postpone from there.]

Worked like a charm.

Where do you get all these Search tricks I've never knew existed?

Here's the best thread here for ideas:

http://www.rememberthemilk.com/forums/help/590/

And also take a look at all of the operators to get an idea of what can be done:

http://www.rememberthemilk.com/help/answers/search/advanced.rtm

I refer back to that last one often.

Many thanks.

Ran, you inspired me to post a list of useful searches on the RTM blog (I've been meaning to do this for a while now). Thanks for your contributions and help on the forums! :)