Going completely from memory -- I think someone at Macworld wrote an article on Expresscard slot SSDs/OS X. Or maybe I'm thinking of Amazon.com reviews.
Which might be a good idea... check out the Expresscard SSD reviews on amazon, et al. I think there were some end user reviews that gave real world feedback on using those drives.
Lion itself is under 8GB, but you have to consider things like safari caches etc that eventually dirty up the /library folder. Toss in a few key apps and 48GB could fill up quickly.
As long as you had a secondary drive for main storage etc it could be a good solution as a boot drive.
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