Since the current donation addresses were created by the 0.1 version and I'm assuming you're using callbacks to trigger the recalc would it be correct to assume that these old addresses which weren't set up with a callback URL will never sync up? Do I need to wipe all of the existing addresses and settings and reinstall the plugin? If so, I know you haven't implemented a one-click solution for that yet, but what do I need to manually delete from where to make that all happen?
Callbacks were created by the v0.1 with just empty function handling them which was doing nothing after the callback - with the intent to implement it in v0.2 and be fully compatible. I am however reviewing the v0.1 code and it seems that the callback URL was missing one of the parameters that are now required and callbacks created by the v0.1 might not be compatible with v0.2, sorry for that. The only effect might be that donations won't be detected and displayed correctly, but they still go to your destination address. I'll release next version around the weekend and will probably include feature to re-create all existing per-post addresses (e.g. to change destination address to forward coins to) so there's no need to manually deleting the current addresses (you can do that and new addresses will be created immediately, but then tips to old addresses will never be included in the stats; re-creating addresses using the feature will preserve previous addresses as well so their tips will be included in the stats).
This is a great widget and of course I were aware of it. It serves different purpose though and I think both widgets will be useful - just for different people/usage cases.
How does it deal with sites that have thousands of posts when the blockchain.info wallet has a 400 addresses limit?
As enmaku already wrote, the plugin doesn't use blockchain.info wallet at all. It uses
Receive Payments API which doesn't have such limits and is “designed to monitor millions of addresses”. One of advantages of using such api instead of managing a wallet is that there are no private keys stored in Wordpress or in the plugin at all - which is important considering Wordpress vulnerabilities history. Plugin descriptions still need some more info about how it works and I'll probably write some description or FAQ soon.
I already have some planned features collected from feedback which are both quite useful and quick to implement, so I'll be releasing next version probably around the weekend.