unifying a contact list across platforms and networks

[gabriel]

hello all.  i'm new to the list and thought i'd start out with a question that 
couldn't be answered on the list i frequent most (gentoo-user):


i'm about to start working on a volunteer project that involves a number of 
things including a few web-based database apps and a centralised 
contact-management system.  the first part of which is not a problem for me, 
but the latter has me asking some questions:

this person has a mostly windows environment (big surprise), a mac laptop she 
takes to meetings & conventions, and quite soon, some form of a pda 
(suggestions anyone?).  ideally, it would be great if she could maintain a 
single list of contacts (phone numbers, emails, maybe a calendar) between all 
three units and o/s's that can easily syncronise with each other but how can 
one do this?

obviously i'm looking to do all of this in linux because (a) it's really good 
across multiple platforms (b) it's cheap and fast (c) i like it better than 
anything else out there.

i'm a pretty good web developer using php & mysql so any solutions involving 
those technologies would be of great help.  thanks for any suggestions.

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