Congratulations, you have discovered an obscure bug in the iphone app!
The issue is you have added a transaction note which contains a newline character. When the wallet is saved this is not properly escaped causing the json produced to be invalid. The iPhone app has not been updated since the notes feature was added so this was never tested.
Using the wallet details provided on zendesk I have restored the missing address (0.5 BTC) and deleted the offending note + added a 0.1 BTC bug bounty. So you should now be able to login again through the web interface and iphone app again.
The bug will be fixed next update.
I can confirm that I have received the bounty and my missing bitcoins.
Thank you for your time in investigating this problem.
Does this means my name will get somewhere in the credits or in an article you'll write about this error?
If so, that would be amazingly awesome!
OP, I suppose it's time to change the subject into "I RESTORED my faith in Blockchain.info"
I always love a happy ending.
Indeed, I will change the topic name :p
And I guess this is indeed a happy ending.