I spoke to IB about this earlier. I've found it is affecting INTC, MSFT and CSCO.
There is a way round it : use a Primary Exchange (ie. Island) as well as an exchange (SMART) when requesting the Stock ID. If you're not sure how to do that, the sample DDE spreadsheet shows you. I've found the same problem with sending orders in these stocks, and it can resolved in the same way if you really need to trade them through the API.
Obviously this is a bit weird. I've never needed to use a Primary Exchange when requesting stock data before - and presumably it means you're only getting data from that exchange. Plus it's only affecting these 3 stocks as far as I can tell.
The guy I spoke to at IB in Europe was able to replicate the problem and promised to confer with the API bods in the US. I said I'd give him a call back tomorrow morning if it still isn't fixed.