1) There are several themes out there to change the UI they can be found on maemo.org and some in posts here. To remove Contacts or Booksmark you have to replace it with something else. Dont think there is a way to just remove it. To replace one of them you can use Personal Menu. Its in the Extras Repo or you can find it on this site.

Personal Menu itT Post

2)KDE is really cool not really a new os but it runs over the OS and looks cool. Also you can get Mac Os can't remember which one running on it as well as a few more.

KDE itT Forum Section


3) Havn't Heard

4)I have an N810 but I use the case that comes it it for now, going to order new case soon.

Also you will want to reflash to OS2008

Edit: Added a couple Links