In September, the Strange Horizons fiction department received 457 valid submissions, making September our 9th-highest-volume month ever. That's an average of a little over 15 stories a day, very slightly higher than August's daily average.
The week starting September 13 brought in 127 submissions, the seventh-highest-volume week we've ever had. Note that that was the week that the Scalzis did their donation-matching thing; I suspect we got a bunch of stories that week from people who hadn't previously heard of us. From September 1 to 13, we averaged 14.7 stories a day; from the 14th (when Scalzi posted) through the 19th, we averaged nearly 19 stories a day. We also had three other odd peak days at various times of the month, with over 20 stories apiece, that don't seem to have been connected to the Scalzi thing.
Comparison to recent years: September of 2008 saw 395 subs (a bit over 13/day). September of 2007 got only 324 subs (under 11/day). So submission volume has gone up by 16% in the past year, and has gone up by over 40% since two years ago. (That's a slightly misleading statement, given the Scalzi thing, but there are always different circumstances each year, so in some sense no two months are comparable.)