I think we'll find that basic Vostroyan Guardsmen are going to be plastic while the Firstborn, officers, spec weapons and the like will be metal. Heavy weapons I'd think would be all plastic to. The talk of a new tank though was debunked in the rumor mill, though it did start to sound as though the Ragnarok was going to be released (Chimera hull with a boxy turret, bit like a KV-1 Russian tank from WWII) before it was scraped.
Cheers, Romanus
Edit: Just looking over the release schedule and see that you can get Quote: Imperial Guard Vostroyan Troopers $12.00, so that now makes me think they'll all be metal.