Wikipedia is usually an extremely accurate source of information about airports. (Perhaps the type of people who are aircraft spotters are also the type of people who contribute to Wikipedia?). According to the relevant page ALL flights to Grenoble–Isère Airport are 'seasonal':
http://en.wikipedia.org/wiki/Grenoble%E2%80%93Is%C3%A8re_Airport
That appears to be confirmed by this page from the airport's official website:
http://www.grenoble-airport.com/en/destinations/schedules-regular-lines
To get there in the summer, travel to Lyon by Eurostar/TGV or using one of these flights:
BA ex Heathrow
easyJet ex Gatwick
easyJet ex Luton
There are direct trains from Lyon Part Dieu rail station to Grenoble (about 1h 25m, £18.00) and from Lyon-Saint Exupery Airport to Grenoble (about 1h 10m, £16.50), with an hourly service in both cases.
Links:
http://www.britishairways.com
http://www.easyjet.com/en/
http://uk.voyages-sncf.com