upgclient works fine for me on /dev/ttyS0 (kernel 2.4.9-ac9, Debian woody).

I usually use Ethernet for emptool, but I do remember successfully using serial for some small uploads (mainly just to prove to myself that it works).

I gave up on USB, though - it would normally start off okay, but hang after a couple of MB. Haven't tried it on 2.0b3 yet.