Any way to create a unique id to send in soapUI requests?

I don't know if you can live with the delay that this would introduce (at least 1 ms per call to the function) or if 2.5 supports this or if it is really correct (how's that for a disclaimer!), but you could try to put the following in your project Load Script:

class IdGen { 
 static def synchronized getId() {
        sleep(1)
        return  new java.util.Date().time
    }
}

project.metaClass.IdGen = new IdGen()

What I think the getId method should do is wait for 1 ms before returning the current time in ms, and since it is synchronized, each thread should wait it's turn to go through the method thus ensuring a unique time. Then when you want a unique ID in a request, you can put in the following:

${=context.testCase.testSuite.project.IdGen.getId()}

Of course if you had a thousand threads that could introduce at least a 1 second delay for a thread to get an id in the worst case.

Good luck.