I just saw a Mycelium wallet misbehave heavily: a transaction was shown as sending only the fee, not the rest. However, on the block chain, it was sent. Mycelium balance was then out of sync with the block chain.
Didn't happen on my device, so I don't have more details right now. But naturally I'm suspecting mutated transactions to be involved.
Erm, somewhat bad timing with this whole bitcoin-wide attack thing. The guys may be on their way to Berlin as we speak, so probably won't get a chance to look at things till later. I asked them for feedback on whether Mycelium is affected by this thing. Will let you know soon as I hear back.
While the malleability 'attack' is raging all wallets (including bitcoin-qt) are affected, but in a minimal way.
This gets a bit technical: When you send a transaction you (most of the time) send change back to one of your own addresses. This is pretty normal and happens all the time. Because wallets may choose to spend this change (especially if the wallet is running low) a new transaction (created before your previous transaction confirmed), may reference outputs that are not valid because of the malleability attach. Gah... this is hard to explain in few words...
In other words: If you are sending transactions in rapid succession before your previous transactions confirm you risk that a second transaction gets invalid, as in "will never confirm". This will continue to be an issue until the malleability 'attack' subsides or the Bitcoin protocol disallows malleable transactions.