I'm asking and answering this here, becase I couldn't believe that there was no quick search result for this. (I got the answer by calling AppleCare.)
Situation: Migrating data from MacBook running Snow Leopard (OSX 10.6.8) to MacBook Air running Lion, using Migration Assistant via local WiFi network.
Problem: Migration begins, initial time estimate "About 7 hours..." Then progress bar shows no further progress, and time estimate increases... and increases... and increases...
1) You need to download an update to the Snow Leopard Migration Assistant - http://support.apple.com/kb/DL1420 and install it on the Snow Leopard Mac. Why this is not featured prominently anywhere in Support, nor has a notification been incorporated in the Lion Migration Assistant, I don't know. (What would Steve say? I mean, really!)
• Do not let either computer go to sleep while you are attempting to migrate. To disable sleep:
1. Choose System Preferences from the Apple () menu.
2. Choose Energy Saver from the View menu.
3. Adjust the "Computer sleep:" slider to Never.
Now, you could argue that 2) should be obvious, but wouldn't it be nice to have a reminder somewhere? Just so you don't have to either re-start the migration, or keep hitting the spacebars to keep the machines awake?