As with every new toy, it has to be explored, and here are some tips for the Sony Ericsson k750i (especially on a mac):

IRC: A nice little IRC Java Midlet: jmirc (via resmo)

SSH: A (most presumably not so useful but working) SSH Midlet: midPssh (via adi)

Semacode: The semacode program works on the k750 with the built in cam. Also not of much use, but has some potential.

You can use the k750i as remote “keyboard”, but the built in schemes are of no use on the mac. Nice ones are available at creativebits for iTunes, DVD Player and Keynote (works also with SlideML :) ), even more on MacTechNews (VLC, iPhoto, QuickTime).

For getting the GPRS connection work with Bluetooth on the Mac, you should get the scripts from Ross Barkman and read the instructions.

If you use the USB connection cable, your Mac will kernel panic after you unmount and remove the cable
 There's a fix available here.

Sending SMS from the built-in Address Book does not work out of the box (yet). Here's the instruction of how to add that.

That's it for the time being, if I discover more, I'll post it.

Update: A nice little page about some of the hints I posted above and much more (in german)