To answer my own question somewhat, I checked VersionTracker, and found that product cycles have tended to be in the year-and-a-half to two-year range over the course of the last few versions. Version 9 was first released in July 2007, so theoretically, a version 10 could be around the corner.
And I assume that the official answer will be "we do not comment on upcoming product plans" :)
To confirm your assumption, FileMaker does not comment about future releases.
I personally hope there isn't a new version any time soon as I, like many others would prefer that feature requests that keep coming up on all the forums out there are finally put to bed & added to the system.
If we have to wait 2 years to get all the feature requests included this would be worth the wait.
However in Corporate business money is king so the more 'new' features made available through chargeable upgrades is what is going to continue to happen.
As a side, forget 10.0 we are currently 9.03, I'm guessing we might see a 9.1 or 9.5 before a 10.0, even then I'm also guessing that the numbering system may even disappear so we may see something like Filemaker X or FMX.