Here's a temporary workaround for this issue (although I've been scripting my forms to work in the manner described above intentionally for a while now, I personally like it better than centered both directions).
Place an "Include HTML" component on your forms, configured as such:
Keep in mind that this may differ depending on the theme you've selected or created for your forms; I'm not sure that the object ID "tableMain" is consistent across the themes. I tested this workaround with about 7 different random themes and each one worked consistently, including "no theme".
Also, to note, we could make this auto-detect the correct "margin-left" and "margin-top" values with javascript and avoid needing to adjust the code above for your specific height/width, but this is the easiest method for a temporary workaround that I have come up with.
Assuming all your forms are the same size, once you've created one of these components, you should be able to copy/paste onto each form and it should work.
If you experience the FOUC phenomenon because of the CSS load times, check this post:
https://www-secure.symantec.com/connect/articles/workflow-css-fouc-and-you
This should work whether you've elected to center the forms or not.