I had previously written about using lo4net from Sharepoint
by including all the configuration settings in the code.

This certainly works, and its very easy.

It would be good to be able to use a standard log4net configuration
file though. In fact that would be great.

And you can.

FileInfo configFile = new FileInfo(_LOG4NET_CONFIGURATIONFILE);
XmlConfigurator.Configure(configFile);

It is that simple.

In the code above _LOG4NET_CONFIGURATIONFILE is a configuration setting I read from the Sharepoint configuration file, but you can hardcode it instead.           

You have to make sure that you put the configuration file in a location that Sharepoint can read from .

One such place is

C:\Inetpub\wwwroot\wss\VirtualDirectories\80

I find it tidy to keep the log4net file next to the web.config file as well.

You can also read from the web.config file that Sharepoint uses with this code:

Configuration config = null;
config = WebConfigurationManager.OpenWebConfiguration("/", "Sharepoint - 80");