Solved!
Javier Kohen wrote:

my guess is that gtkpod is compiled against version .26 for 64 bits, but you only have the 32-bit library for that version.

I suppose Javier was right, although maybe some more investigation would be nice... so let me write some info and the solution.
This is my architecture from uname -a:
Linux merlinux 3.0-ARCH #1 SMP PREEMPT Wed Aug 17 21:55:57 CEST 2011 x86_64 Intel(R) Core(TM)2 Quad CPU Q9400 @ 2.66GHz GenuineIntel GNU/Linux
This is the gtkpod binary which got called before the solution:
file `which gtkpod`:
/usr/local/bin/gtkpod: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.27, BuildID[sha1]=0xa36d237faab8ee584adfd3358edaff2068d1b748, not stripped
And these were the versions of the modules involved in the problem before applying the solution:
file `which /usr/lib/libgnutls*`
/usr/lib/libgnutls-extra.so.28.0.1:   ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked, BuildID[sha1]=0xf4f5b366b0f307a581415ea231b403b0283c9621, stripped
/usr/lib/libgnutls-openssl.so.27.0.0: ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked, BuildID[sha1]=0xc9d5d8a06e295abf1f963a52b4a4382931faf80c, stripped
/usr/lib/libgnutls.so.28.0.1:         ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked, BuildID[sha1]=0xa4da4f2d7e5220e653894ae91a0808064f8c0b63, stripped
/usr/lib/libgnutlsxx.so.28.0.0:       ELF 64-bit LSB shared object, x86-64, version 1 (SYSV), dynamically linked, BuildID[sha1]=0x9b68cd9c8547edcf20a1e75f0e5f10a4a56cd5c7, stripped

file `which /usr/lib32/libgnutls*`
/usr/lib32/libgnutls-extra.so.26.18.10: ELF 32-bit LSB shared object, Intel 80386, version 1 (SYSV), dynamically linked, stripped
/usr/lib32/libgnutls-openssl.so.27.0.0: ELF 32-bit LSB shared object, Intel 80386, version 1 (SYSV), dynamically linked, stripped
/usr/lib32/libgnutls.so.26.18.10:       ELF 32-bit LSB shared object, Intel 80386, version 1 (SYSV), dynamically linked, stripped
/usr/lib32/libgnutlsxx.so.27.0.0:       ELF 32-bit LSB shared object, Intel 80386, version 1 (SYSV), dynamically linked, stripped

The gnutls packages I've got installed are (shown by pacman -Qs gnutls):
local/gnutls 3.0.1-1
local/lib32-gnutls 2.12.2-1

Before describing what solved the situation, I apologize!
You can point at me your finger, because I found the solution before I could do your tests...
so I have the solution, but really I don't know much about why it worked!
:(
I simply compiled and installed libgpod and gtkpod from sourceforge repository (0.8.2 libgpod and 2.1.0 gtkpod).
They both recognized x86_64 as architecture in ./configure phase.
After that, i run gtkpod expecting the "libgnutls.so.26 not found" error, but the program just launched fine.........
Maybe all this confusion was caused by the fact that, before putting the sourceforge libgpod, I had the Arch repos version installed: so trying to put the git version of gtkpod with libgpod from repos caused the error.
What can I say, sorry for all the time I stole to you, I promise to be more careful next time.
Thank you all again,
Riccardo Merlotti