According to this CFO.com story, the long awaited US SEC decision has finally materialised today: XBRL disclosures are to become mandatory in the US for firms with a market capitalisation larger than USD 5 bio from the closing of the current fiscal year. All other firms will have to follow suit by 2010 (first XBRL filings). Non-US domiciled firms listed on US markets and reporting using IFRS will have to start disclosing in XBRL starting 2011. This is quite an ambitious time frame, indeed, but we don't complain as the use of interactive data by investors can only take off once the data is actually available.
Update: Here is the official press release.