The raspberry router of an earlier post turned out to work well, but it definitely needs an enclosure. The fast & easy solution - put everything inside a plastic Smart Store box! (The same kind of box we modified for storing Legos.) The box contains a USB hard drive, as well as the Pi and GSM modem, so we can use the Pi as a file server.
The Pi, the network switch, the USB hub and the hard drive each has its own power supply, so we included a power strip in the box. The many power sources take up a lot of space in our current system. If all the devices could share one, the system would shrink considerably.
Each machine has its own LED, and at night, the whole thing looks like a box of holiday decorations.
The box is kept slightly ajar for the cables going into the box, which seems to provide enough ventilation as well. Measuring the raspberry's CPU temperature with
cat /sys/class/thermal/thermal_zone0/tempconsistently reports around 46 C.
This setup turned into a surprisingly nice media player as well, once I connected our old TV set to it. We have no keyboard connected to the Pi, so we just ssh to it and run the omxplayer. I plan to give XBMC a try later, but getting it set up on a standard raspbian systems seems rather involved.
The current setup is somewhat temporary - in the future, I hope to put everything inside the case of a desktop PC, to function as a power conserving and silent always-on server, independent of the main PC.