First things first you will need a serial data cable to connect the Psion to the PC, if you have a 3 or 3a then the Psion 3-link cable is your only option but if like me you have the 3c or MX then you have several choices:

a) buy the official PsiWin cable/software bundle
b) buy the cable on it's own if you can find someone that will sell you one
c) make your own

I ended up buying a 3c cable from Widget Software. They are now also available from Serial Leads for 15 UKP. There are instructions here if you want to make your own 3c data cable.

The next thing you need to do is download P3nfs (readme) and install it on your *nix box.
P3nfsd will allow you to mount your Psion as you would any other device as the example below shows:

[nsc@liberator nsc]$ ls /psion.stand/m/
agn           jot           scr 
apd           opd           spr  
app           opl           tmp
autoexec.bat  opo           wdr 
bin           pic           wld
dat           psistack      wrd
email3        sav           wve
idf           sco
[nsc@liberator nsc]$ 

Once you have installed P3nfsd on the PC you need to install the corresponding Psion program Nfsc which will have come with P3nfsd. You need to get this software installed on the Psion somehow. You can do this using the Psions built in comms app. Set the protocol to ymodem 1 k and use minicom on the Linux box to send the nfsc.app file using the ymodem protocol.
I cheated and used a Windows PC and the free DS3AEPL software, alternatively you can download PsiWin 2.1 from Psion although it is spread over 13 floppy disk sized files whereas DS3AEPL is considerably smaller at 788K.

Once you have the software installed on both machines and the cable connected to a spare serial port you can get down to business:

Start Nfsc on the Psion, go to the UNIX menu and make sure the Protocol is set to Terminal + New p3nfsd , check the serial paramaters and make sure the baud rate is set to 57600 baud.

On the PC type p3nfsd -user nsc -tty /dev/ttyS1 -speed 57600 where nsc is my user name and ttyS1 is the com port the Psion is connected to.

You should now see a message similar to the below:

p3nfsd: version 5.3, using /dev/ttyS1 (57600), mounting on
/psion.stand/mnt
p3nfsd: to stop the server do "ls /psion.stand/mnt/exit".
(pid 3454) 

That's it, your psion is now mounted on /psion.stand/

You don't need to stop p3nfsd if you disconnect your Psion you can just unplug it and walk off, you can quit Nfsc on the Psion if you want and just restart when you need to reconnect. If you do need to stop P3nfsd you can do so by typing ls /psion.stand/mnt/exit.

You might want to look at arb's Psion software for a suite of conversion utilities for reading Psion files on unix.

Have fun!