Yes I've just setup a tunnel between two 6.0b3 boxes and it seems to working fine. There are some X509 binaries compiled for this kernel You can download them here
http://download.freeswan.ca/freeswan-x509/RedHat-RPMs/1.99/2.4.20-18.7/ Install the freeswan binaries first then install dev-info next using --nodeps.
I did have an error come up on one of the sites about rp_filter = 1 should be 0 so I changed it and the tunnel came up staright away.
[root@server root]# service ipsec restart
ipsec_setup: Stopping FreeS/WAN IPsec...
ipsec_setup: Starting FreeS/WAN IPsec 1.99...
ipsec_setup: Using /lib/modules/2.4.20-18.7smp/kernel/net/ipsec/ipsec.o
ipsec_setup: WARNING: eth1 has route filtering turned on, KLIPS may not work
ipsec_setup: (/proc/sys/net/ipv4/conf/eth1/rp_filter = 1', should be 0)
[root@server root]# echo "0" > /proc/sys/net/ipv4/conf/eth1/rp_filter
[root@server root]# cat /proc/sys/net/ipv4/conf/eth1/rp_filter
0
[root@server root]# service ipsec restart
ipsec_setup: Stopping FreeS/WAN IPsec...
ipsec_setup: Starting FreeS/WAN IPsec 1.99...
ipsec_setup: Using /lib/modules/2.4.20-18.7smp/kernel/net/ipsec/ipsec.o
[root@server root]# rpm -q devinfo-freeswan
devinfo-freeswan-1.99-6sme56
[root@server root]# rpm -q freeswan
freeswan-1.99_x509_0.9.15_2.4.20_18.7-1
[root@server root]# rpm -q freeswan-module
freeswan-module-1.99_x509_0.9.15_2.4.20_18.7-1
HTH Regards Lloyd