C'mon dude, you know you only had to call me!!!
I got this set up and working about a week or two ago...
- I'm running Real VNC Free on the HTPC (it seems to update the remote desktop faster than TightVNC - possibly better options).
- I had to manually change my network's IP address range from the 192.168.0.1 to the 192.168.1.1 range, as apparently PortableVNC (or PSPVNC in our case) doesn't like 0s (zeros) in any IP addresses!
- I changed all PCs in my network to Static IP addresses, and manually entered a Static IP address and the rest of the IP setup into the PSP (it's suggested that this is necessary to get it to work). I then went back into the router and allowed a DHCP pool from 192.168.1.101 through to 192.168.1.200 - I need DHCP to still allow my work laptop to connect wirelessly.
- I allowed ports ranged 5800 to 5900 on the router
- I turned off the Windows Firewall on the HTPC! - It was only after doing this did it successfully connect to the VNC host on the HTPC.
Now, this all worked quite well until I took the PSP outside, then the desktop session on it started locking up etc. due to poor wireless signal strength out there (a bit of structure to get through). The standard wireless antennas that comes with my Netgear DG834G was a 2dBi, so I bought a replacement 5dBi antenna and presto! Asside from a couple of grey spots out there I can use my PSP to run a desktop session on my HTPC anywhere in my outdoor entertaining area. I personally go with Windows Media Player out there as the reason I wanted to do this was to access my entire CD collection (which I have on a 2nd HDD in the HTPC - losslessly compressed of course) and play it out there, and I reckon the MCE interface has more overhead and takes more to poll to the remote desktop.
Anyway - I've just finished a weekend where yesterday I had a BBQ out there for lunch with the inlaws and BBQ dinner with my side of our combined family, the whole time picking and playing various CDs from an almost 300 strong collection - even controlling the volume (via the Volume slider on Media Player!). Awesome!
I hope the pics are OK

.