This is interesting, and something you might consider if you own a qualifying hard drive-less Xbox 360 Core or Arcade machine:

An Xbox 360 Hard Drive is required to unlock all the great features of the new Xbox 360 Experience so upgrade using this special offer today and get online!

In order to connect to Xbox LIVE and enjoy everything the new Xbox Experience has to offer, you will need an Xbox 360 Hard Drive. While you can connect to Xbox LIVE using an Xbox 360 Memory Unit with 128MB of available space, you will not be able to enjoy all the great features like Xbox LIVE Primetime and video downloads.

Enter your information to see if you’re eligible for the Memory Upgrade Program.

So... I did this on the Arcade unit I have and received the following message:

Congratulations! According to our records you have the necessary hardware and storage space to enjoy the new Xbox Experience.

Instantly stream movies from Netflix*, download the latest High Def TV shows and Hollywood hits^ or play classics like PAC-MAN, Galaga Legions, Bejeweled 2 and UNO.

Express your personality with a customizable, animated Avatar that allows you to literally jump into the world of Xbox LIVE. Plus, transform every entertainment experience into a shared experience using text, Instant Messenger, voice or video chat and the new LIVE Party to connect your friends, family and the entertainment you love.

We’ll see you online!

Granted, there is a hard drive attached to it right now, but that’s one that came from another (now red-ringed) machine. I own a 64 MB MMU, but that was also purchased separately. What is Microsoft using to determine that I don’t need the HDD upgrade? My understanding here is that most HDD-less Xbox 360s would qualify except for the newer Core/Arcade systems that come with a 256 MB MMU. No?