Microchip also created a chip (enc28j60) that has *you* do the Protocol Work in software. Guido Socher http://www.tuxgraphics.org/electronics/200606/article06061.shtml
got it to work with Atmel chips and NuElectronics got Guido's solution converted to the Arduino... As you can see if you look for it... The PHYSICAL layer speed is why these ETHERNET chips need to run between 25 and 50 mhz.
With just the Physical & Data Layer work done by the Microchip part, you are left with precious little room for code to make the Arduino work for you since the Transport and Protocol layer support needs to be in the Atmega chip. It pretty much mandates a Atmega328 to leave enough for you to work with.
The WIZ chip used in the official Ethernet shield does all the Physical and Data Layer work as well as all the (TCP/IP) Transport and Protocol layer work in the chip. Leaving room for your code.
I've worked with Ethernet since 1980... for one of the companies that helped invent Ethernet... working as a network guy. This "Ethernet" stuff is actually stuff I know pretty well. I wish you could understand that your question about "just attaching to arduino pins" is the equivalent to wanting to know which wire you should move under the hood of your car that will make your car give you 50 more miles per gallon of fuel.
If it was that easy...
Here is a blog about a guy who got the Miccrochip part to work with a mega168. http://members.home.nl/bzijlstra/software/examples/enc28j60.htm
Oh, and SLIP is a protocol for Modems and terminal ports not Ethernet ... You have a serial port... I'm sure you can find an RFC on SLIP you can work from.