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From the PS3 manual online:

"If the router in use does not support UPnP, you must set the router's port forwarding to allow communication to the PS3™ system from the Internet. The port number that is used by remote play is TCP: 9293. For information about setting this option, refer to the instructions supplied with the router."

I found that even with UPNP enabled forwarding this port is still necessary. Otherwise the PS3 will power up but the PSP will not complete the initial connection to it. Reconnecting via local LAN will work after that, but not across the internet.