A similar question was asked about the Cybook at Baen's Bar (E-Book Reader, First Time in Conference thread). Since I can't work out how to link to it from their "improved" forum viewer (which is terrible - I may give up participating at Baen's Bar because it is so bad), here is the reply post by delphidb:
Here's Laurent Picard's (Bookeen) response:
The display operating temperature range is 0..50 °C (32..122 °F).
The display storage temperature range is -25..70 °C (-13..158 °F).
This is a significantly better than standard LCD screens.
The battery charging temperature range is 0..40 °C (32..104 °F).
Below 41 °F, batteries can present a shorter operating lifetime.
In a nutshell, though it can run outside this range, the recommended operating temperature range is 5..35 °C (41..95 °F).
I have personally used my Cybook last summer in the mountainside, under direct sunlight, with temperatures above 100 °F over few days and it ran just fine.
I assume the screen specs will apply to all Vizplex screens, but the battery specs may be Cybook (or PVI reference design) specific.