Eddy,
If you have an xDSL line with static IP's on a /27, then PBI/SBC will setup the DNS as follows. In this example W is the base IP of the network (ie: 0,8,16,24,32,40,48, etc.) and (W+n) should just be a number and not have parentheses or a plus!
PCI/SBC will add the following to their zone files...
W.X.Y.Z.in-addr.arpa. IN NS <your-nameservers>
W.X.Y.Z.in-addr.arpa. IN NS <your-nameservers>
W.X.Y.Z.in-addr.arpa. IN NS <your-nameservers>
In my case they did NOT list PBI/SBC as a "NS" for that specific zone, hence it always comes over to my boxes.
Then PBI/SBC will add this in their zone files...
(W+0).X.Y.Z.in-addr.arpa. IN CNAME (W+0).W.X.Y.Z.in-addr.arpa.
(W+1).X.Y.Z.in-addr.arpa. IN CNAME (W+1).W.X.Y.Z.in-addr.arpa.
(W+2).X.Y.Z.in-addr.arpa. IN CNAME (W+2).W.X.Y.Z.in-addr.arpa.
(W+3).X.Y.Z.in-addr.arpa. IN CNAME (W+3).W.X.Y.Z.in-addr.arpa.
(W+4).X.Y.Z.in-addr.arpa. IN CNAME (W+4).W.X.Y.Z.in-addr.arpa.
(W+5).X.Y.Z.in-addr.arpa. IN CNAME (W+5).W.X.Y.Z.in-addr.arpa.
(W+6).X.Y.Z.in-addr.arpa. IN CNAME (W+6).W.X.Y.Z.in-addr.arpa.
PBI/SBC did not do the W+7 entry for me but they did do the W+0 entry. 
That all said, you just need to add one zone "W.X.Y.Z.in-addr.arpa" on your side.
Why is this confusing? Because if you got the same email as I did... they didn't even come close to explaining it this way and hence why your worried about the recurse on the NS's.
Contact email address I have in my files for PBI/SBC DNS are...
"HARPER, LACONTRIA (SBIS)" <lh6712@sbc.com>
DESC Central <DESCCentral@sbis.sbc.com>
Note that I don't work for SBC, I just use an xDSL line at home.
Martin
