One of the things I mentioned is that there are implementations of Open XML right now, and more will come in the future. An example is Gnumeric, a free fast and accurate (according to them :) spreadsheet tool that you can get for free.
Within our Dutch committee the chairman has raised the notion that Gnumeric wasn't able to open any of the sample spreadsheets I had sent, even the simplest one. They are the same spreadsheets I use for the Open XML workshops, download them here. While I find that hard to believe, I just went with it. Now it appears that he hasn't been trying too hard to make it work. According to the test you can read about here, GNumeric had no issues with a simple spreadsheet saved as SpreadsheetML from Excel. Funny to see that while one remains 'totally' unable, the other has no issues. Makes me wonder how he tried it in the first place...
Now not all of SpreadsheetML is supported already in Gnumeric, but that's a non-issue to me. To this day there is still no 100% faithful ODF implementation as well, and the market place hasn't collapsed because of it.