Sorry to anyone who's been coming in looking for THE FINAL RECKONING: I was overtaken by a load of work last week. Here we go, though - I'll do the Guardian first, then the Observer, then the GRAND TOTALS.
First up, a reminder of what the actual gender split of the Guardian readership looks like:

Now let's look at how many letters published on the Guardian letters page between 17th April and 16th May 2008 were written by men, and how many by women:

And expressed as percentages:

Now, I would say that even taking into account possible margins of error (I had a comment a few posts back from a woman called Jamie - but if I'd seen her in the Guardian I would have automatically counted her as a man), there is a lot of discrepancy between the gender split of the Guardian readership and the gender split of Guardian letter-writers who have their letters published.
This doesn't of course, automatically mean that the Guardian letters editor is biased against female correspondents. There are several possible reasons for the discrepancy. Here they are again:
1. Fewer women write letters to the editor in the first place.
This one seems the most likely (especially as the letters editor of the Times Higher Educational supplement
said in January that 95%* of the letters written to the publication are by men), but it does open up the field for a load of sub-theories about why this should be the case.
2. Women do write letters to the editor, but only on a narrow range of topics.These tend to be traditionally 'female' topics like domestic violence, abortion, anorexia and equal rights in the workplace.
3. Women do write letters to the editor, but are less likely to be selected for publication.Precedence is granted to letters from public figures, who tend to be men. In the absence of a letter from a public figure, precedence is automatically granted to male correspondents because they are - consciously or subconsciously - assumed by the (male) letters editor to have more authority or higher social status.
4. Women do write letters to the editor on a broad range of topics, but are more likely to be selected for publication when they're writing about 'female' issues.Women are seen to be authorities on typically female topics, but not on topics of more general interest (unless the letter-writer is also a public figure or a senior academic working in the subject area).
**NEW!** 5. Very few women write letters to the editor, but proportionately more of them get their letters published.The Guardian letters editor, being (we assume) a liberal, left-leaning sort of individual, feels he must represent the views of female readers on the letters page, despite the fact that very few women actually write in. Therefore, he chooses a disproportionately large number of female-authored letters for publication in an attempt to provide at least some sort of balance.
Hopefully before long I'll have an answer to all this.