I am having the same issues you are. Actually, from what I can tell (using version WP 2.01), anything that gets imported gets imported as user "webmaster", and not based on who you are logged in as.
If indeed you are seeing a situation where it imports the posts as the author you are logged in as, then I would suggest choppin the RSS file up into different files based on author. (Eg, rss_me, rss_bob, rs_jenn, etc). Tedious, yes? But if it works, it woks, eh? As I said though, I wasn't able to get this to work.
Also, if you manage to get your RSS into another system, then you can import it from there. I was able to bring in posts from Blogger where the author was correct. But unfortunatly, Blogger does not allow a mass import.
Let me know if you have any luck. I'm still trying to figure this out as well.