Yes.
Once Mycelium gets to version 1.0 (1.0 goal is within 2013) BitcoinSpinner will get retired. The import functionality in Mycelium however will remain.
Note that BitcoinSpinner allows you to backup in two ways.
1. Settings -> Backup (the one that you can also restore in BitcoinSpinner)
2. Settings -> Advanced -> Export (the one that gives you a standard SIPA formatted private key)
Option 2 is the one that is broadly supported across wallets, so that would be the most portable way of backing up your BitcoinSpinner key. Mycelium can import both.

Implementing BIP 32 and creating keys is the easy part. Where it really starts getting tricky is when have to track track the unspent outputs and transaction history of an ever increasing key set. Some have suggested a moving window of keys to track, and that will work as long as no one sends funds to an address outside the window. Bitsofproof is working on such a solution, and it is going to be interesting to see how it works out. My guess is that a lot of confusion will arise when funds seemingly disappear when they are received outside the window...
You can amended this by doing a complete scan once in a while. This however is where it gets really expensive. If you have your own dedicated powerful host running Armory this is not a problem, but I don't think it scales to thousands of wallets.

I have a development suggestion. How about adding a CoinJoin mixing option to Mycelium wallet? That would be an excellent feature. Similar to what Blockchain.info is doing with their sharedcoin trustless mixing right now. What do you guys think about this idea? These sort of features are a must if we want to continue to protect the privacy of Bitcoin users.

Aren't you guys already running Mycelium servers?

Btw, I didn't look into CoinJoin too closely (hadn't had time to run it myself yet), but doesn't it need Tor to be really anonymous?

I don't think its lack of resources that is tripping you up. One of the core devs has talked about using a used sony phone for mycelium. You can see it in action in the play store video.

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

Will the testnet version be dropped at some time (like it happened with Bitcoin spinner, IIRC), or will be mantained along the production one (like Bitcoin Wallet)? I think it would be good idea to keep it, not only for testing purpose, but It could be useful to show peoples how bitcoin works without messing up with the real thing.

I have ten active (read-only) keys. Mycelium will not let me add an eleventh key. (The "+"button is dead.) Is there any setting where I can increase this limit?

Cool. That solves my problem. Thanks.