You should be able to, yes.
In WinXP: Right-click your network icon on your desktop, and select Properties.
In the network window, you should see two iBurst connections - the PPPOE connection, and a LAN connection. Right-click the LAN connection icon, and select Properties. There's a sharing option under the Advanced tab. Check that, and select OK. You'll be prompted that Window will reconfigure your LAN IP.
I've never used configured a PS3, but if it's anything like the Xbox you should have a network settings option screen somewhere. There, set your IP to be automatically assigned, although I've found (on Xbox) that manually defining iBurst's DNS addresses works better than acquiring them automatically. For the gateway IP, use 192.168.0.1.
That should work.
If you're using any firewall software, you might have to forward the PSN service ports to your DMZ.
If you're using a different version of Windows, however, then ... I dunno.
[EDIT] Oh, heh. I just noticed you're using Win7. Eh, I can't imagine it's really all that different.