We've had two sources corroborate that Apple isn't putting together a home server just yet but instead is going to update Time Machine at some point in the very near future (Macworld?). The update is relatively minor and the biggest feature is that it will allow the Time Machine to cache software updates for all Macs on the network. This functionality exists in Leopard Server and is nice to have in a home device - especially as updates get bigger and bigger.
Current users should be able to take advantage of this, you guessed it - with a software update.
We'd still like to see Apple move into the home server market soon and will be keeping our ears to the tracks to see if anything is materializing.