True or not, this is a persistent rumour, going all the way back to the 60s. Wikipedia, naturally, has the full rundown (http://en.wikipedia.org/wiki/Soviet_space_program_conspiracy...).

But in brief: this is just one of a range cosmonaut conspiracy theories which put the 'moon landings were faked' crowd to shame. They're also much more widely believed than their US counterparts (not without reason). This one maybe not so much - but a decent percentage of Russians believe Yuri Gagarin was bumped off by the authorities.

Also, if you hadn't noticed, the article's from way back in 2001.