I have samba configured with the following smb.conf file:

workgroup = mydomain
netbios name = servername
security = domain
printcap name = cups
disable spoolss = yes
show add printer wizard = no
idmap uid = 15000-20000
idmap gid = 15000-20000
winbind use default domain = yes
use sendfile = yes
printing = cups

comment = My new share
path = /export/myshare
valid users = myusername
read only = no

If I try to:
mount -t smbfs -o username=myusername,workgroup=mydomain
//servername/myshare /mnt/samba
After typing in my password I see:
25744: tree connect failed: ERRDOS - ERRnoaccess (Access denied)
SMB connection failed

I also get an access denied error when trying to "net use" the share from a
windows machine

but, I can:
smbclient -L servername -U myusername
After typing password I see:

Domain=[MYDOMAIN] OS=[UNIX] Server=[Samba 3.0.7-1.3E.1]

Sharename Type Comment
----------- ------ ----------
myshare disk my new share
IPC$ IPC IPC Service (Samba 3.0.7-1.3E.1)
ADMIN$ IPC IPC Service (Samba 3.0.7-1.3E.1)
Domain=[MYDOMAIN] OS=[UNIX] Server=[Samba 3.0.7-1.3E.1]
Server Comment
-------- -----------
mypdc PDC
servername samba 3.0.7-1.E3.1

Workgroup Master
----------- ----------
mydomain mypdc

I can also run the following commands successfully
wbinfo -u
wbinfo -g
getent passwd myusername
getent groups

Anyone have any idea what is going on here?