I think I have the same problem on FreeBSD7. Mail-notification segfaults right after fetching headers from gmail. It seems to be locale-related (everything works fine with LANG=C, normally I'm using pl_PL.UTF-8).

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x8190f00 (LWP 100263)]
strlen () at /usr/src/lib/libc/i386/string/strlen.S:48
48		repne				/* search! */
Current language:  auto; currently asm
(gdb) bt
#0  strlen () at /usr/src/lib/libc/i386/string/strlen.S:48
#1  0x2910af32 in __vfprintf (fp=0xbf6fbdbc, fmt0=0x8217bb9 "nag\ufffd\202ówek pobrany pomy\ufffd\233lnie (%s bajtów)", ap=0xbf6fbebc "")
    at /usr/src/lib/libc/stdio/vfprintf.c:1052
#2  0x29065a5b in vasprintf (str=0xbf6fbe74, fmt=0x8217bb9 "nag\ufffd\202ówek pobrany pomy\ufffd\233lnie (%s bajtów)", ap=0xbf6fbeb8 "^\001")
    at /usr/src/lib/libc/stdio/vasprintf.c:59
#3  0x28cd965c in IA__g_vasprintf (string=0xbf6fbe74, format=0x8217bb9 "nag\ufffd\202ówek pobrany pomy\ufffd\233lnie (%s bajtów)", args=0xbf6fbeb8 "^\001")
    at gprintf.c:313
#4  0x28cbe596 in IA__g_strdup_vprintf (format=0x8217bb9 "nag\ufffd\202ówek pobrany pomy\ufffd\233lnie (%s bajtów)", args=0xbf6fbeb8 "^\001") at gstrfuncs.c:218
#5  0x08061e8e in mn_mailbox_notice (self=0x81c80c8, format=0x8217bb9 "nag\ufffd\202ówek pobrany pomy\ufffd\233lnie (%s bajtów)") at mn-mailbox.gob:1151
#6  0x0808bd16 in ___5_mn_gmail_mailbox_authenticated_check (mailbox=0x81c80c8) at mn-gmail-mailbox.gob:190
#7  0x08088c26 in mn_authenticated_mailbox_authenticated_check (self=0x81c80c8) at mn-authenticated-mailbox.c:644
#8  0x08088bc1 in mn_authenticated_mailbox_check_thread_cb (self=0x81c80c8) at mn-authenticated-mailbox.gob:227
#9  0x28cc9850 in g_thread_create_proxy (data=0x81e73a0) at gthread.c:635
#10 0x29030a69 in pthread_getprio () from /lib/libthr.so.3
#11 0x00000000 in ?? ()

(gdb) bt full -10
#2  0x29065a5b in vasprintf (str=0xbf6fbe74, fmt=0x8217bb9 "nag\ufffd\202ówek pobrany pomy\ufffd\233lnie (%s bajtów)", ap=0xbf6fbeb8 "^\001")
    at /usr/src/lib/libc/stdio/vasprintf.c:59
	ret = Variable "ret" is not available.
(gdb) bt full -9
#3  0x28cd965c in IA__g_vasprintf (string=0xbf6fbe74, format=0x8217bb9 "nag\ufffd\202ówek pobrany pomy\ufffd\233lnie (%s bajtów)", args=0xbf6fbeb8 "^\001")
    at gprintf.c:313
	len = 0
	__PRETTY_FUNCTION__ = "IA__g_vasprintf"
#4  0x28cbe596 in IA__g_strdup_vprintf (format=0x8217bb9 "nag\ufffd\202ówek pobrany pomy\ufffd\233lnie (%s bajtów)", args=0xbf6fbeb8 "^\001") at gstrfuncs.c:218
	string = (gchar *) 0x0
#5  0x08061e8e in mn_mailbox_notice (self=0x81c80c8, format=0x8217bb9 "nag\ufffd\202ówek pobrany pomy\ufffd\233lnie (%s bajtów)") at mn-mailbox.gob:1151
	_mn_strdup_vprintf_args = 0xbf6fbeb8 "^\001"
	message = 0x6 <Error reading address 0x6: Bad address>
#6  0x0808bd16 in ___5_mn_gmail_mailbox_authenticated_check (mailbox=0x81c80c8) at mn-gmail-mailbox.gob:190
	self = (Self *) 0x81c80c8
	atom_size = 350
	node = (xmlNode *) 0x8190f00
	result = GNOME_VFS_OK
	atom = 0x826a040 "<?xml version=\"1.0\" encoding=\"UTF-8\"?>\n<feed version=\"0.3\" xmlns=\"http://purl.org/atom/ns#\">\n<title>Gmail - Inbox for <email address hidden></title>\n<tagline>New messages in your Gmail Inbox</tagline>\n<f"...
	doc = (xmlDoc *) 0x811b2c0
#7  0x08088c26 in mn_authenticated_mailbox_authenticated_check (self=0x81c80c8) at mn-authenticated-mailbox.c:644
	klass = (MNAuthenticatedMailboxClass *) 0x8139f20
#8  0x08088bc1 in mn_authenticated_mailbox_check_thread_cb (self=0x81c80c8) at mn-authenticated-mailbox.gob:227
No locals.
#9  0x28cc9850 in g_thread_create_proxy (data=0x81e73a0) at gthread.c:635
	thread = (GRealThread *) 0x81e73a0
	__PRETTY_FUNCTION__ = "g_thread_create_proxy"
#10 0x29030a69 in pthread_getprio () from /lib/libthr.so.3
No symbol table info available.
#11 0x00000000 in ?? ()
No symbol table info available.