useradd should not be used for samba/ldap. use the ldap scripts that come with samba - on some distros you might have to search for them but they should be there somewhere. adding computer to domain has always been a little flakey but check your root/administrator ldap entry is working and is being recognised as a domain admin and your computer entry should add ok. older versions of samba used to have a bug that reported an error but if you look the computer entry is actually added to the tree.
i find these packages are well built and up-to-date, often much more so than distro packages:
Enterprise Samba: samba-enterprise
samba updates frequently and often fix many bugs that often don't get backported to distro packages.