This is apparently Phase 2 of a beta program to extend the textual content of AdWords, with the rationale being better CTR figures form their Phase 1 results. This change ONLY impacts the description field, permitting up to 200 chars, so long as the description remains broadly similar to the existing creative. The submission deadline is 8th August.
Given the rest of the contents of the email, I believe that ads that have "gone North" are being selected for this phase, as the main difference in display will be on top strip ads. RHS ads will continue to look the same but will trail.... like so at the current character cutoff. According to the email, the longer ads won't be visible in your account, but will be reported on separately