... No worries though; it's not like we don't have the private keys ourselves.

Exactly! If you are not the only one who controls your private keys then you don't have any bitcoins.
Now let me explain what caused the missing confirmations.
The short version:
One of our super nodes got stuck on block 265458 due to a bug in our software. It has been fixed and everything is working as expected.
The long version:
We have node that continuously runs against the Bitcoin TEST network, and some days ago we observed that it got stuck on a block (
http://blockexplorer.com/testnet/b/119058) which encodes script chunks in a non-standard way that we did not anticipate. This is the first block on the test network that has this behavior, and at that time it had never occurred on the production network (>4 years long block chain). A fix was made and verified to work against the test network.
The fix was also applied to one of our nodes on the production network (wanted to verify that it worked as expected before applying it everywhere). Yesterday a block with a similar non-standard script chunk encoding appeared on the production network. Because of that the unpatched node got stuck while the patched node continued to hum away happily.
Because the Mycelium wallet chooses a random node whenever it is restarted some users observed that no blocks got confirmed while others saw everything work as expected.
There is a brief discussion about my observations here:
https://bt.irlbtc.com/view/315509.0