developed for MOSS 2007? I've found four solutions so far, but I'm pretty sure
there are more:
1. Use System.Web.HttpContext.Current.User.Identity.Name, like in any
ASP.Net web application
2. Use System.Security.Principal.WindowsIdentity.GetCurrent().Name
3. Have the Current User Filter send the current user to your web part,
and (of course) make your web part a consumer of that value.
SPUser currentUser = web.CurrentUser;