There's a type of SSDs (Solid State Drives) called mSATA that takes the form of a mini-PCI Express card (LINK for more info).
For $200 (September, 2011) you can purchase an 80gB Intel mSATA drive (LINK to NewEgg). It's a minuscule thing!
I have just installed it on a Lenovo ThinkPad X1. I'm running a standard 160gB Intel SSD on the computer and now have a secondary drive in the form of the mSATA installed under the keyboard where the mobile broadband card would go. Unfortunately, it's either the mSATA or the mobile card, not both.
The user guide for the X1 has a pretty good explanation of how to do the install. One step you can skip is disconnecting the keyboard. You'll have to remove the keyboard but no need to disconnect it.
Remember that you have to format the drive after you install it and also that if you don't have it as the boot drive, make sure it's set as the second drive in the list for startup.