I think the reason I was getting the doldrums last September was because normal people tend to take vacation over July and August, so e-mails slow down in general. The corollary to this is that they pick back up again in September.
I'm rather glad I wrote that post, if for nothing else than to know that at the time I had about 650-660 unread e-mails in my inbox, and ten months on I've whittled it down to 620, only a handful of which are less than a month old. My gmail space usage has also increased to over 50% of capacity (from 35% in September 2009), presumably from a larger number of attachments. (Curse Gmail for not letting you delete e-mail attachments!!!)
I could probably even get it down further if I looked at the older ones and archived or deleted the messages that are definitely no longer relevant, but at a glance, all the ones on the first page of 50 are things I'd still like to attend to (reading reports, filing files, reminding me to post stuff on websites, etc.)
The important thing is seeing that 620 number being lower than what it was before. It means progress.