By default, what is maintained by the 0.8 blockchain engine is:

• All blocks
• A database with all block headers, and the positions on disk for each block
• A database that represents all unspent transaction outputs (UTXOs), indexed by txid, at the current tip of the block chain
• Undo files that allow rewinding the effects of blocks on this set of UTXOs.

Nothing related to addresses or balances exists at this level. That is just a client-side representation of the data. The balance of a wallet is the sum of the values of the UTXOs that are spendable with the set of keys in the wallet. You could define the balance of an address as the sum of the values of the UTXOs assigned to a script matching a given address (and this is what several sites do), but presenting it this way is quite confusing - it makes many people think that Bitcoin transactions transfer value from one address to another. This is not true, they consume specific outputs of a previous transaction, and merge and split those into new outputs.

The -txindex option you mention, adds one more thing that is maintained by the validation engine:

• A database with the positions on disk for each transaction, indexed by txid.

This was added to make the getrawtransaction command work with historical transactions, but again, this has nothing to do with addresses. A database that indexes the UTXO set by address, or even the entire block chain history by address would be possible, but it would be relatively expensive to maintain, and shouldn't be necessary for normal operations (though it is quite useful for debugging...).

Where such things do belong, is in the wallet. The wallet as presented by the reference client does not use "balance of an address" - it rather sees addresses as entry points into the wallet, but manages the coins in the entire wallet together, regardless of what address they were last sent to. This also confuses people who are used to look at blockexplorer-like websites, when they realize that for example change is always sent to a fresh address.

So, to give a real answer: if you really want to track a balance, you need the wallet interface. Right now, this is quite unflexible, but there are plans to improve this situation soon. We'll hopefully soon get support for multiple wallets, and watch-only wallets, where you can just add addresses you'd like to track, without having their keys.

1. import all address using "importaddress"
2. you can get balance using "listunspent", but you must sum all unspent amount per an address.

The concept of an "address balance" is based on confusion.

The way Bitcoin is designed, there are three different concepts related: addresses, wallets, and accounts. Wallets are comprised of one or more accounts, each having a balance (which may even be negative). The wallet as a whole has a balance totalling the sum of all accounts (which must always be positive).

Addresses are merely entry points to a wallet; they do not have balances, and should only ever* be used once. Addresses are associated with an account: when coins are received on the address, it credits that account - note that this is a one-way association: the account is not associated with the address, nor the coin received; its balance is simply increased by the amount received. When you spend bitcoins, your wallet chooses coins to spend at (more or less) random, with no concern for how they were received, and will usually use a completely new invisible address internally with itself, that you will not see as an end-user. Overall, these are some of the reasons why it makes little sense to talk about an "address balance".

* While it is fairly common that people reuse addresses for deposits or donations, doing so is harmful to yourself and others using Bitcoin in a number of ways which are outside the scope of this question.