Familiar (located at
http://familiar.handhelds.org/) is, as far as I know, the main linux distribution for PocketPC mobile devices (I'm using it on my Ipaq H5450, and I'm quite happy with it). They do not have a released version for Axims yet, but work is in progress. For further information on the status of linux on the Axim X5, see their wiki page on it:
http://handhelds.org/moin/moin.cgi/DellAximX5. Maybe letting them know that you'd like to use linux on your Axim might give them some more motivation to get it working
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Dell Axim x5 400MHZ I want to put Linux on it!!!!
