There has been a bit of an issue with a full x64 MOSS farm, mainly due to the lack of ifilters for x64, the obvious (and also, importantly, tested and supported) workaround being the use 32bit for the Index server and x64 elsewhere.
The most common third party ifilter needed by clients (in my experiance) is for PDF documents, Adobe were very slow to respond and deliver an x64 ifilter (in fact they still haven't) but now have a publishing work around toi running v8.1 ifilter on x64, see this: http://labs.adobe.com/wiki/index.php/PDF_iFilter_8_-_64-bit_Support
there is no mention of the threading issue that existed previously, hopefully this is resolved now but our testing may highlight this as still existing, will post if this is the case...