Before getting my rMBP, I used an i7 Mac Book Air 4GB with an external OWC Express USB 2.0 bus-powered enclosure with an OCZ 240GB Vertex 3 SSD. Worked well on the plane, in the hotel, and when connected to my 24" LED monitors at the office.
I ran FCP X, Motion, PP and AE very respectably.
Should run you around $1,700 for the newer 2GHz i7, 8GB MBA (you could save $100 there with the i5 CPU) and around $200 for the external SSD and you get the bullet-proof Apple quality.
Tim
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Tim Jones
CTO - TOLIS Group, Inc.
http://www.productionbackup.com
BRU ... because it's the RESTORE that matters!