Sony Ericsson T610
This great piece of pocket technology can cooperate with Linux easily. First of all, I've tried only serial cable and Bluetooth connections as far I don't own any IrDA hardware (except some homemade crap which doesn't work anyway :).
Sending and recieving stuff
The main idea behind this is to use OBEX protocol. The easiest (and actually the only) way is to use OBEX Push. OBEX FTP is implemented somehow different in the cellphone and standard Linux-based obexftp doesn't work - it can only list directories. If anybody wants to fix this, I can provide remote account on my machine with T610 attached to it.
The phone supports SyncML, unfortunatelly only using WBXML (binary XML). There're not many products which support WBXML.
Very cool stuff
Due to some special AT commands it's possible to use all GUI elements of T610 using Bluetooth. You can use several softwares for this; unfortunatelly most of them require some extra stuff - Python, Ruby or any other crap language around. If you just wanna experience the technology, try using btcontrold, which doesn't require anything except C compiler. Controlling XMMS through your cellphone just rocks, believe me!
Check out the theme (modified a little bit by me) for T610.