Generated blocks show up as mixed_debit until they "mature", which is around 120 confirmations. After maturity, you are credited the 50 BTC. This is standard policy for all generated blocks in bitcoin.
I've never seen the point of this. Ostensibly it's to stop the coins being spent until they're definitely part of the block chain.
After a block is created, the maturation time of 120 blocks is to make absolutely sure the block is part of the main chain before it can be spent. Your node isn't doing anything with the block during that time, just waiting for other blocks to be added after yours. You don't have to be online during that time.
The thing is, if you allowed it to be spent immediately it could only be spent in a branch of the block chain that's includes it's own creation. If that branch is not the longest then both the transaction creating it and the transactions spending it will be rolled back anyway! There's no justification why they should be viewed any more skeptically than a normal transaction especially as they're so small. A 10000BTC transaction doesn't take any longer to mature than a 0.01BTC transaction.
ByteCoin