The release notes contain full details. They have set things up such that it will work with AMD syntax, and still work with the legacy dojo.require/dojo.provide syntax.
If you are using the require/provide syntax, you need to register a module path prior to requiring dojox.cometd:
dojo.registerModulePath("org", "../org"); dojo.require("dojox.cometd");
For AMD applications, either use the “tlmSiblingsOfDojo: true” configuration attribute, or use a configuration object specifying the “packages” field and map that to the location of cometd.
CometD 2.5.x ships updated demos that you can use as examples:
* Dojo 1.7 AMD-style demo: the Reload demo, the Chat demo
* Dojo 1.7 non-AMD demo: the Echo demo, the TimeSync demo
Please report any bugs you find to the CometD issue tracking system.