Although best done with
mortise and tenon (easily done with your router and bandsaw and a jig), you
can also use dowel joinery. this eliminates weak miter joints and gives you
a quality, traditional bed.

Thanks for your responses guys. My initial idea was to use pillars
instead of bedposts and attach the siderails to those. I don't want a
traditional siderails+bedpost bed. side view:

--
| |
| | side rail sits on the rebate on the outside of pillar
(7.8x7.8) below:
| |
--
|-------|
| | ~4 inch high rebate
---| |
| |
| |
|----------|

they'd be glued+dowel joined (the mitres as well)
Would that be a strong enough joint, or shall I rethink my design?

thanks:
Marton