From the Accumulo User mailing list on Oct 10, 2012, John V says:
Depending on the Instance your using, the client may hit HDFS first to get some instance information. Then either instance will hit Zookeeper to get the root tablet location. It will then scan that to find the appropriate !METADATA tablet, but will aggressively cache everything. The client will then scan (with heavy caching again) the !METADATA tablets necessary to find the tablet(s) for the range of writes. And then the writes will occur to appropriate tablets discovered in the !METADATA scan.