There are a number of CWM installable .zip files being distributed that break Clockworkmod in a way that Rom Manager will not fix. This .zip file will reinstall the B&N bootloader (to enable the recovery buttons) and install ClockworkMod on your internal memory. This .zip will only help if you have installed a "flashable" .zip and can no longer get into CWM by holding N + Power on boot.
This file can be installed with IOMonster's bootable CWM to re-enable the ability to use CWM without an SD card.
How to use:
1) Create a bootable CWM SD card from the link above.
2) Mount the SD card on your computer and copy the RecoveryFix.zip file to it.
3) Boot the Nook into CWM from the SD card.
4) Choose "install zip from sdcard"
5) Choose "choose zip from sdcard"
6) Browse to the RecoveryFix.zip and select it.
7) Choose "Yes - Install RecoveryFix.zip"
CWM should now work by booting and holding the N + Power buttons.
The bootloader (u-boot.bin) file included with the stock image has code to look for the recovery key sequence on boot. It also contains the "Touch the Future of Reading" splash screen. Some of the update .zip releases include a custom uboot.bin (to change the splash screen) that does not look for the recovery keys...so there's no way to boot into recovery without an SD.
Currently the only automated way to put the correct files back is by using one of the stock recovery .zips. These take the entire system back to stock though. All this .zip does is replace the stock u-boot.bin file, CWM uRecRam and CWM uRecImg files on the /boot partition. This can also be easily fixed by booting any image from an SD card and copying these files manually.