What is /private/var/db/dyld?

Well, Rosetta is the OS X emulation engine that allows older Power PC (PPC) based software to run on Intel based Macs. Being a disk space freak can get you into serious trouble with OS X. It's a Unix variant and various caches are important to its proper operation. I would suggest you download one of the cache utility programs like Leopard Cache Cleaner, Tiger Cache Cleaner, Onyx, Cocktail. Use them to clean your caches instead of doing it yourself. They were written to use built in Unix commands to do their thing.

Since you don't know, it's best you stay out of any hidden folder, /System, and usually /Library.

See these for things you can do:

Freeing space on your Mac OS X startup disk
Freeing Up Hard Disk Space

Running maintenance routines is critical to keeping the drive clear of unneccesary files:

http://www.macworld.com/2005/01/features/preventmacdisasters/index.php
http://www.thexlab.com/faqs/maintscripts.html

That directory has to do with prebinding and shared caches (relevant to application launches). And no, it is not safe to delete (I'm not sure what would happen, but I know it would not be a simple fix.)

Don't go deleting stuff around the system, especially in hidden directories.

If Cocktail didn't touch these files you shouldn't either, in my humble opinion. Major applications that require Rosetta are Office X, Office 2004, Quicken, older versions of Photoshop. You can find out if an application uses Rosetta by performing a Get Info on the app. Under Kind look for Kind:Application (Power PC). That means it needs Rosetta to run on an Intel Mac.