The only thing I can think of is check your firewall/router settings.

Install Little Snitch and watch how iChat tries to connect to the screen sharing session in the network monitor. If it shows a port, try forwarding the port in both your software and hardware firewalls and see if that works. If you've done so on your end and you still have the issue, have your peer (the person you are trying to connect to) do the same.

Screen sharing might be off in either/both your and your peers system preferences. You can find this under the Sharing option in System Preferences, and make sure Screen Sharing is turned on.

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Mac OS X Leopard 10.5.8 (9L30) | Chameleon 2.0 RC3 | Intel Pentium 4 3Ghz 800Mhz FSB HyperThread 1MB L2 cache SSE3 1MB L2 cache works, cosmetic display of 512KB L2 cache | Asus P4V8X-MX VIA Chipset, VIA-VT8237 Southbridge | AC97 VIA8237 | Dual 1GB 333Mhz DDR SDRAM | nVidia PNY GeForce 7600GS 512MB 8x AGP DVI/TV-Out/VGA [NVinject 0.2.1; QE/CI/QuartzGL/Rotation] | Darwin Kernel Version 9.7.0: Sun Jun 14 20:48:28 IST 2009; Voodoo 2.0 Intel alpha3 :xnu-1228.12.14/BUILD/obj/RELEASE_I386 i386