Doing a quick survey on a few books, I have seen that there is no consensus. I have books (non-fiction, technical books) that have Acknowledgements before and after the Contents page. However, I did see a larger number inclined towards including the Acknowledgment page after the Contents page even though there are books which have it before the Contents.
I do not see any obvious advantage or disadvantage of either of the methods. When writing a thesis one usually has a style guide as to where to include which part of the thesis and hence it is not a question in that case.
As per books, I would say that it is based on the publishing house and the style guide that they would like to follow. Usually a particular publishing house will have a fixed standard style which they apply to all of their books (for example, Blackwell Science usually has a fixed font style and overall design). Hence, I would say that you can go ahead and do it the way you would prefer it.
Getting It Published: A Guide for Scholars and Anyone Else Serious about Serious Books (Chicago Guides to Writing, Editing, and Publishing) talks about the front matter of the books. However, at no point have they clearly mention explicitly mention anything about the arrangement of the front matter.
The Chicago Guides to Writing, Editing, and Publishing have Acknowledgements after the TOC. However, they do not mention anything about the order.
The Complete Guide to Self-Publishing: Everything You Need to Know to Write, Publish, Promote and Sell Your Own Book does not mention anything about the ordering of the various content of the front matter but has Acknowledgements before the TOC.
These and others seem to highlight that the order is more about the style guide selected rather than a matter of strict consistency. Given the heterogeneity, one might as well go ahead and do it the way they want it to be done.