Two FreeBSD slices, 32bit and 64bit

Hello,

I bought an athlon64 processor recently and everything went just fine
using my old installation of FreeBSD in 32bit mode.
Then I decided to give amd64 a try, deleted some unneeded linux-slices
and created a new FreeBSD slice with a single partition. The current
layout looks like this:

     Device Boot    Start       End    Blocks   Id  System
/dev/ad2s1   *         1         7     56196    6  FAT16
/dev/ad2s2             8        20    104422+  83  Linux
/dev/ad2s3            21      3668  29302560   a5  FreeBSD
/dev/ad2s4          3669     14593  87755062+   5  Extended
/dev/ad2s5          3669      9812  49351648+  a5  FreeBSD
/dev/ad2s6          9813     14592  38395318+   7  HPFS/NTFS

My 32bit installation is on ad2s3, my experimental 64bit on ad2s5. The
partitions are:

# /dev/ad2s3:
8 partitions:
#        size   offset    fstype   [fsize bsize bps/cpg]
   a: 57605104       16    4.2BSD     2048 16384 28552
   b:  1000000 57605120      swap
   c: 58605120        0    unused        0     0


# /dev/ad2s5:
8 partitions:
#        size   offset    fstype   [fsize bsize bps/cpg]
   a: 98703281       16    4.2BSD     2048 16384 28552
   c: 98703297        0    unused        0     0


To make things even funnier, I use grub installed on ad2s2 as my
bootmanager, booting my 32bit FreeBSD on ad2s3 with:

rootnoverify (hd0,2)
chainloader +1
boot

After installing the amd64 version of FreeBSD 6-BETA1 via
cross-compiling on ad2d5a, I ran a "bsdlabel -B ad2s5".
Now, when I try to boot ad2s5 via grub using

rootnoverify (hd0,4)
chainloader +1
boot

the boot loader gives an error about an invalid slice and hangs. It
looks like this:

<registers being dumped>
BTX halted
Invalid slice
>> FreeBSD/i386 BOOT
Default: 0:ad(0,i)   <-- why i?
boot:

Booting ad2s3, unloading the kernel, loading the 64bit kernel and then
booting works, this is what I'm doing now. But this can't be the right
way :-/

Anyone has any ideas why the boot loader is trying to access partition
i? Any help how to fix things appreciated, I'm really clueless here :o(


-- 
Fortune cookie of the hour:
Politicians are the same all over.  They promise to build a bridge even
where there is no river.
		-- Nikita Khrushchev