I need to expose a Web Service that lets me create and manage work
items in RTC, so that I can trigger those create / modify tasks from
a Business Process (BPEL) by just invoking that service.
Could any of you point me to the best way of doing that, is there an
API, Java maybe for that, or does the Jazz server come with REST
services that can handle those kind of tasks? Could you also provide
me with a link to some useful documentation or tutorials on the
subject please?

There is currently no officially supported Restful HTTP API for work
items. This is planned for 2.0.

You can create work items using plain Java, however. Please see

https://jazz.net/wiki/bin/view/Main/ClientLibrarySetup
https://jazz.net/wiki/bin/view/Main/ProgrammaticWorkItemCreation

An example snippet:

IWorkItemWorkingCopyManager workingCopyManager=
workItemClient.getWorkItemWorkingCopyManager();
IWorkItemHandle handle= workingCopyManager.connectNew(fgDefectType,
null);
WorkItemWorkingCopy workingCopy=
workingCopyManager.getWorkingCopy(handle);

workingCopy.getWorkItem().setHTMLSummary(XMLString.createFromPlainText(getName()));
workingCopy.save(null);

--
Regards,
Patrick
Jazz Work Item Team