From: SmugBlog: Don MacAskill » Blog Archive » Nasty Bug: Safari doesn’t cache stuff.

• If your computer has less than 1GB of RAM, Safari fails to cache items larger than 104,857 bytes.
• If your computer has more than 1GB of RAM, Safari failes to cache items larger than 209,715 bytes.
• JPEGs, at least, are temporarily cached in RAM. Whew. But upon browser restart, you’ll see they didn’t make it to the disk cache, so you have to get them again.
• Other objects, like SWFs or videos, though, don’t even make it to the RAM cache, let alone disk. Load the same SWF back-to-back, and you’ve just transfered the bytes twice. Ugh.

Pretty nasty stuff Apple!