I'd say its a relatively safe bet that there will be a new version of iWorks, less certain is how Apple will release it. Apple is in a transition period between retail software (i.e., software packages such as iWorks 08, iWorks 09, etc) and a completely online distribution model. There is no reason why they couldn't keep using the iWorksyear approach but they could just as well move to an upgrade approach in much the same way as Lion is an upgrade to Snow Leopard.
There are no safe bets here, but if you need an office solution now, I'd go ahead and get iWorks 09. Personally I'd go through the app store even if its a few dollars more than buying the full package on Amazon for ebay for the following reasons.
The ability to redownload software to all your Mac's is a pretty good deal
You can pick and choose. For example I need a spreadsheet solution for the MBA I just picked up. I could install Office 2010/Win 7/Paralells but I dont wantb to kill off 30 or so GB for the limited use I will have for Windows. I don't have any real use for presentation software. I'll probably pick up Pages as well but generally prefer using a simple markdown editor for most of my writing.
There is always a chance that if Apple release a new iWork version any time soon they would cut recent purchasers a break. I think that the break is far more likely if you purchased iWork via the app store than a retail package but then again no safe bets.