It looks like Microsoft has started to get in touch with developers behind the 5% of apps which "do not run satisfactorily on 256 MB devices", just as promised. The intention behind these e-mails is to give developers time to profile their apps, optimise them, and then test out any improvements on the new 256MB emulator.
How do we know this? It turns out the WPCentral app is in that 5% of apps, here's the e-mail I received:
Interestingly, from this e-mail it looks like any apps which were identified as being one of the 5% will automatically be excluded from the Marketplace for users on one of the new lower-end devices. Another useful item we can report is that the final version of the new SDK is targeted for release in March.
If you're a developer who just received the very same e-mail I highly recommend you jump straight to Justin Angel's guide from Nokia's developer portal to start profiling your app. You'll also find tips to make your app Tango compatible here. It's exactly what I did and you might just have the same issue that I do: a lot of pictures at a higher quality then they need to be.