your question is easy to get lots of responses, but difficult to come up with any truths... mainly because much of this question is very very qualitatively different for different people.
where to start with some of the questions...
do you need low latency - do multi-track recordings, or simply audio down in independent takes?
how many channels do you need to record at any 1 time?
here are some of the items that i have been looking at, personally...
Grace Design Lunatec V3 - with toslink optical digital option.
http://www.gracedesign.com/products/V3/lunatecV3.htm
can be battery powered. great preamps and A/D converter. no integrated monitoring option. only does 2 channels. the MBP has an optical digital input, so the optional optical output model could be hooked up directly to the MBP without any intermediate interface
Apogee Mini-Me USB option
http://www.apogeedigital.com/products/miniseries.php
can be battery powered. great preamps and A/D converter. integrated monitoring option. only does 2 channels and up to 48KHz sampling rate. built in compression / limiting can be useful to some, but not others.
Apogee also has the firewire option card coming up and the reason that I suggested the Mini-Me with USB option vs. FW, is that you could also get a Mini-DAC equipped with FW and then when you need to, can plug the digital output of the Mini-Me into the Mini-DAC and use the FW interface (with up to 96k up / 192k down) to the computer... so would basically have the choice of USB or FW with this combo.
obviously the above options are very limited and hardly qualify as "interfaces" in the traditional sense, unless you have very minimal I/O requirements, but my application requires only 2 channels and battery power for me is a must.
my idea of portable might also be different than yours. that expresscard solution looks good if you can afford it.