Today I tried to install freeBSD on one of my partitions.
I have partionned the whole disk with linux fdisk.
I installed slackware 13.37.0 on /dev/sda1
These are empty (reiserfs-formatted):
/dev/sda2, /dev/sda3, /dev/sda5, /dev/sda6
and /dev/sda7 is a swap partition

When I got to the (freeBSD) fdisk stage, I selected the third partition
(which turned out to be ad4s3 in BSD lingo) and changed its type
to a BSD type (165). Then I pressed 'q' and tried to proceed to the
next stage. But I kept getting messages about chunks to starting
on a track boundary. Something like
I pressed ok, thinking that those are just warnings,
but I couldn't proceed to the next stage.
I'm trying to figure out what to do next (resize partitions
with gparted, or install freeBSD on first partition...)
Has anyone encountered the same problem?

Yes I have...though not in BSD but in my own attempts...
I usually get around this by carefully calculating the boundries or by partitioning with sizes rarther than other measures...I'd accept the start as default (fdisk knows where what chunk starts) and enter a M/G value, I'd enter something line 8G for 8 Gigabyte as size.
I noticed that fdisk figures the rest out by itself...

By the way, but this is a personal observation, I try to keep one OS per box, keeps conflicts like what you're having out of the door

Good luck

Thor

That's what I actually did (with the linux fdisk), accepting start as default and entering sizes as
G values: 20G for most partitions ...

This time I used fdisk from linux and set /dev/sda2 as a freebsd type partition.
Then I launched the freebsd installation, selected that partition (called ad4 in
freebsd, I can't figure their naming scheme), created two slices (one for /, the
other for swap). Despite the warnings about track boundaries, this time I was
able to proceed and installed successfully freebsd.

Glad you've got it "licked"

Happy Linuxin'!

Thor