Download the proper full ping utility from the repos (search maemo.org/packages for "ping" and there's too in there that are basically the same thing, and work the same).

Everyone advising the use of -i for ping needs to pay attention to this - the default ping utility that comes with busybox preinstalled on the N900 cannot use the -i flag. It's that badly stripped. The full one is in the repos, though, so you can apt-get it or manually download it if you know the name.

But yeah, I do this on my campus WiFi to keep the connection alive. I found that ping at -i 240 is sufficient to keep the connection going for me. (I keep raising it by a minute when I remember to, but pinging every 4 minutes hasn't caused a connection time-out yet.)