VMware 6.0 will not be released in Aug - Sept. It will be closer to the end of the year. I would not wait for the VCAP6-DCA. VCAP6-DCA cert probably won't be out for 3 or 4 months after that.
The course will help speed up your study time, but the cert can easily be accomplished without it.
Here are all the places that class is being offered online - VMware Education
Here is a link to my experience and some good study resources - http://blog.marccrawford.com/vcap5-dca/
Please do not forget to mark helpful and correct answers.
I have not yet taken the VCAP, but I have been studying for it for the last month+ and I've taken the Optimize & Scale training as well as the vSphere Troubleshooting training course. I think the perfect combo would be the Jason Nash Optimize and Scale videos along with the vSphere Troubleshooting instructor lead training course. When I took it a few months ago it was specific to 5.5, and I thought it was a great course for getting you to think about solving common vSphere issues which I would expect to appear prominently on the VCAP.
Optimize and Scale covered at least 70% of the exam and it was a huge benefit to me passing the DCA on the first go; the books and labs helped a lot. I would get the VDCA550 even if 6 does come out this year the VCP on 6 alone would be April or so next year with VCAP6 about a year out from now even. Considering a lot of places won't upgrade to 6 until a dot release at least it's not a huge factor. The VCAP on any version is a pretty big compliment to your knowledge and having passed the 5 will make you a lot more confident going into the 6 when you go for that.
I would say to go get the cert. Being familiar with one version of VMware (usually) helps with the next test(s); even if 6.0 is not out this year, all the work you put into 5.x will pay off in increased familiarity.
Re: Optimize and Scale, I would say that the class is not necessarily a 1:1 match for VCAP-DCA.
If I were to give advice on preparation for the DCA test, I would say that you should be confident that you could implement ALL the features of VMware version 5.x. You should be able to administrate, or build in the real world, a complete VMware multi-node cluster with integration with Microsoft AD. Be able to fully utilize CLI tools, the vMA, and so on. Get comfortable with all the network and storage features, know how to troubleshoot those features and ensure that what you have scales. It's been over a year since I took the DCA, so many things may have changed, but I think you need to practice, practice, practice with the final goal being that you could fix any problem anywhere in the designed Datacenter.
In the event that you work primarily in a Linux environment, get nice and familiar with Windows. The reverse is less true, but because the vMA is Linux-based you should have a decent grasp of general *nix commands.
I don't believe there is a requirement to use the vMA in the 5.5, it may not even be in the lab environment.
I took Optimize and Scale v5.1 and there was a lot of vMA (management, and performance). Though there were some items in the DCA Test Track for the vMA that were not covered in the Optimize and Scale class (AD integration if I recall) Definitely not 1:1, but I agree with wjlorenzo that it probably helped me pass (I went in really cold so I relied on that course plus my experience and a few pages out of the unoffical official study guide by Jason langer and Josh Coen).
My advice would be for any configuration that you are doing in the DCA exam, make sure you know how to implement, troubleshoot and back out.
For DCA exam does not matter which tools do you use (expect if a couple of tasks where you have to use PowerCLI, but you have only to understand a minimum of this script Language).
I think there was (in the 5.x version) also one task related to vMA (that is available in the environment)... But anyway miss some tasks make still you pass (if you do almost well the others).