I'm jealous; I'm stuck on Windows Mobile 2003 here!
In 2003, and most likely also in 2005, you can change your IP address with this path:
Start -> Settings -> Connections -> Network Cards ->
selected network card -> Use specific IP address.
You can probably change your MAC address by editing the registry. To edit the registry on Windows Mobile, download and install something like
Resco Explorer 2005 or
All-explorer.