The Core Pocket Media Player (TCPMP) has long been the most popular multiformat media player for mobile devices. Although many have migrated over to the for-pay Core Player, TCPMP still is around and still gets a lot of use from power users. Here's one more use: playing YouTube (or any other flash) videos. Plan on instability, thought, it's a v.02 release as of this writing.
Also as of this writing, no plans to add it to Core Player.
It is designed for Pocket PC 2003, Windows Mobile 5.0 and higher. It has two parts: 1. A program to download YouTube movie by PPC. 2. A TCPMP plugin to play flv file directly on PPC (the plugin will never support CorePlayer)