mount filesystem and run a program when hotplugged

[Eugen Leitl]

I've bought me a little (32 MBytes) hotpluggable USB flash stick (a
TrekStor). It mounts fine, but what I'd like to do is to automount it, and
fire up a program (I intend to put my keyring on it) if hotplugged.

The system I'm testing this on is RH 7.3. 

I've been using mount -t vfat /dev/sda /mnt/usbhd to mount it manually and
put diverse entries into /etc/fstab, to no avail. Any suggestions?