I've had three lockups so far in the day I've had the 3G, and have had to do a power-off/reset each time (power slider for ~15-seconds). It happened once when scrolling in the web browser, once after pressing enter to view a full word definition, and once when paging through a PDF.
If I continue to see the problem I'll give Kindle support a call to see what they have to say about it.
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