Splunk can centrally index your log files and provide real time search capabilitys over TB's of data
You can download, install and start using Splunk for free in 5 mins, http://www.splunk.com/download
Furthermore , all of your "machine data" in your multi tier environment can be sent to Splunk and correlated together to give you a really nice single pane of glass transactional view of your production systems.
You can easily tag all the log events in Splunk for each individual client, and then use Splunk's powerful search language to create operational views, dashboards, alerts and reports.
As far as getting the logs into Splunk , there are several options, but I would look at these 2. Both options provide failover / high availabilty in case a tier goes down.
Splunk Universal Forwarder : http://www.splunk.com/download/universalforwarder
SplunkJavaLogging log4j and logback appenders : http://dev.splunk.com/view/community-projects/SP-CAAAED7