I haven't seen much mention of this yet so I wanted to call it out in case folks weren't aware yet. For Office "12", all of the XML functionality will be enabled in all SKUs. In Office 2003, the XML file formats were enabled in all versions, but the support for customer defined schemas was only in the Pro-SKU and not in the standard SKU.
The customer defined schema support was really a valuable feature in Office 2003 as it allowed people to not just rely on the schemas that we defined, but you could actually use your own schemas to add additional meaning to your documents. Anyone who has played around with this functionality has seen the great value here. If you aren't familiar with it, just read through the Intro to Word and Intro to Excel posts I've made over the past couple months. You can also check out the free online labs. The schemas that we defined (WordprocessingML and SpreadsheetML) are really valuable, but it's the customer defined schemas that really give your documents true interoperability with other business processes and systems.
So, for Office "12" all that XML functionality will be available in all versions. If you were at PDC and went to Tristan's talk, you also saw that we are doing a lot of work to make it even easier to leverage your own XML by mapping it into data islands within the documents for true data/view separation.
This is an awesome move that I've really been excited about for awhile (I'm glad I can talk about it now).