I wanted to share my learning curve about this problem for CS3 users and below who use a Canon 5dMk 2.
My previous workflow had been to convert all my CR2 files to Adobe DNG using Lightroom version 2 which works fine. However I have started using DXO Optics Pro v 5+ which is unable at present to read the DNG files.
So I now use DXO to convert the CR2 files into Linear DNGs which I can then use in PS and LR etc. What I didn't realize is that PS is unable to read the native RAW CR2 files from the Canon as the minimum ACR specification is 5.2! CS3 ACR is stuck at 4.6.
If you were only using PS CS3 you would be unable to read your RAW files created using the Canon and so you would need CS4.
Just thought this might be helpful for anyone out there.
I back up and archive all my original CR2 files to a separate hard drive in case I need access to them in the future. Interestingly there is a very technical discussion out there about Linear DNG vs Adobe DNG and the potential merits/ demerits of the two formats.
As I have a large library of Adobe DNG files I would hope that DXO optics Pro is in future able to access these.
DXO optics Pro is doing a magnificently interesting job at optimizing my RAW files and I can recommend this program... but the workflow learning curve can be steep!

No...wrong. You would need to download DNG Converter 5.3 (a free application from Adobe) which can convert the 5D MII raw files that 4.6 and even 3.7 in CS2 and ACR 2.4 in CS. So, no you would _NOT_ need to have CS4 simply to open the raws in CS3 if you convert to DNG. It would be useful if you get you head wrapped around that...

Hey thank you! But I had always been led to believe the 5.2 ACR was not downloadable separately.. I will give it a try.

Downloading ACR 5.3 as a standalone program?
I have just checked the Adobe Web site and unless I misunderstand the language there, I can only download ACR5.3 if I have CS4 installed on my system.
Any advice on this would be most welcome and please point to a weblink if you can where I could download ACR5.2 or 5.3 free to integrate with CS3.
Currently this doesn't (to me) seem possible.
But open to any advice.

Currently it's DNG 5.3. And yes, you can download separately now. Even before when the ACR and DNG were combined into one compressed download file, you could still use the most recent DNG Convert and toss the version of ACR which you couldn't use.

Go to the DNG Product Page. Go to the Downloads in the upper right area and click on your platform. Leads you to the DBG Convert download which also includes the DNG Profiles (that will also work in ACR 4.6).

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I will download DNG 5.3, but that still leaves me with this. I have to convert the CR2 files to DNG in order for Bridge CS3 and Photoshop CS3 to recognize/ see the files and that in turn leaves me with DXO Optics Pro unable to read the DNG files... at least at present.
So on the one hand if I want to keep my native CR2 files to be openable in DXO as the first point of call they are not visible in Bridge CS3: on the other if I convert them with DNG 5.3 then they are not visible or usable with DXO.

Its a workflow choice.