I record on a laptop, so maybe I can offer some insight.
Without the expandablity of desktop machines, you would be limited to a USB device, maybe something like:
http://www.midiman.com/products/m-audio/duo.php
I would recommend staying away from USB devices, just because it is the most limited connection to your PC. There are other options though: Firewire (such as the MOTU 828/896) and PCMCIA (Echo Mona/Layla, RME Hammerfall). It is true that there are a greater variety of devices available for PCI, but there are fine options that work for your laptop. I am quite happy with the Echo laptop devices.
Performance-wise a properly configured desktop will blow away a notebook for recording
I have to respectfully disagree with this statement. I don't see anything lacking in a carefully configured laptop that you can get in a desktop PC.
The primary disadvantage of a laptop is expandability: not only will the laptop cost more than a comparable desktop up front, but as you expand it, it will cost more as well. I found out shortly after getting involved in recording that extra hard drive space is vital, and an external firewire hard drive is both more expensive and more of a hassle than an internal drive for a desktop. Same goes for memory or most any other item you might want to add in the future, assuming you even can add it to the laptop.
If portability is of value to you, don't be worried about your laptop's ability to do the job -- I've never had a problem working with up to 16 tracks simultaneously on mine. If it really isn't necessary though, a desktop will be more cost effective and allow a wider range of sound cards to choose from.
Oh, and there is one other disadvantage to laptops that too few people consider: I've never known anyone to have a desktop PC stolen, but I know at least 5 people personally that have had laptops stolen. After experiencing that kick in the balls once myself, I'd stay miles away from owning a laptop if portability wasn't necessary for me.