Whoops - as of the 1.6.4 update, i can no longer connect.

MacOSX 10.4.6 2GHzX2 G5 4Gb RAM
DSL connection: SBC/Yahoo via Efficient Networks Speedstream 5100 modem

Oops!

This is a known issue. a1k0n's working on it.

Be afraid...be very afraid.

As mentioned here and here try setting the MTU Discovery (under Network) to "Compatibility Mode" and see if that helps.