Datomic [0] uses SQUUIDs [1] (Semi sequential UUIDs) to work around this:

    Many UUID generators produce data that is particularly
    difficult to index, which can cause performance issues 
    creating indexes. To address this, Datomic includes a 
    semi-sequential UUID generator, Peer.squuid. Squuids are 
    valid UUIDs, but unlike purely random UUIDs, they 
    include both a random component and a time component.

[0] http://www.datomic.com/

[1] http://docs.datomic.com/identity.html#sec-6

edit: formatting