It's a bug.
We've quickly fixed it in last beta https://dl.dropboxusercontent.com/u/13064071/plantuml.jar so
[-> Alice : request()
[<-- Alice : ok
is working fine now. However, we are pretty sure that some similar issues involving [-> and ->] are still present, so do not hesitate to post a message here with examples of non working diagrams.
Thanks again for the report!