Here is a video demonstration of JellyBean on Nexus One:
This is based on this post: http://forum.xda-developers.com/showthread.php?t=1762790
I had some issues with the way suggested at xda, so this is the way I managed to install JellyBean on my Nexus One device.
I have no data size issues thanks for a2sd.
1. install BlackRose and hboot.
1.1 ignore eset troygen warning
1.2 use the maximum for system partition (260)
2. Use clockwork recovery to partition the sd.
2.1 Install it via rom manager (free from the market).
2.2 Create the partitions from the recovery (not from windows tools), the first partition is a FAT32 this is your main partition, viewable from windows,
The ext partition should not exceed 1.5G, and will be called (automatically) sd-ext, its not viewable from windows.
The swap partition should be twice the size of your device RAM (1G for Nexus One).
Both ext and swap partitions are not viable from windows, and are only used internally by Android (a2sd).
3. Copy the installations files to your SD card (using your PC).
4. Put the sd in your device, and boot to recovery, boot to recovery and install the OS, reboot normally, you should now have JellyBeans,
see that your SD exists (by connecting to your pc via usb, and mounting the SD).
5. adb to your device, a2sd install (not m2sd), and follow the installation steps. After reboot all apps and data should be on the sdcard.
5.1 in some cases you will need to run a2sd formatext, then run a2sd install (then reboot) more then once.
Enjoy your JellyBean on your Nexus one, for me it works much better then the stock OS.