One caveat; as a filmmaker, I have worked with SAG in the past, employing members for a few movie projects. I have never had a problem with the union, and they were always extremely helpful in securing SAG talent for my films.
Here are the details: SAG and AFTRA's contracts with the studios and producers (AMPTP, Alliance of Motion Picture and Television Producers) expire June 30, 2008 (yes, you read that right - 4 days from today), and there has been zero progress. The issues at hand are contract negotiations including key issues new media and DVD royalties.
There is another big issue which has kept the two unions from sitting back down with the AMPTP: fighting between the two. AFTRA has a potential deal they want to solidify by July, but SAG isn't happy with their terms, and so there is a major battle happening between leadership and members, and there doesn't appear to be much in the way of progress in the past few weeks.
Unlike the WGA strike, which mostly affected a lot of new productions without scripts from moving forward but allowing existing projects with scripts to continue shooting, an actors' strike will absolutely paralyze the industry. No one can work, and a high number of movies and TV shows feature union actors, so everything will be shut down. Not to mention that actors won't do interviews to promote upcoming movies, which will hurt the box office and affect next month's huge San Diego Comic-Con.
So how does this affect current projects, or those in the immediate future? ComingSoon.net posted an article a few weeks ago detailing what's happening with movies. Many films had to start by February or March so they could be wrapped before the potential strike, otherwise they'd have to wait until after a resolution is made - and many big budget movies have already been shelved. Other films, like The Da Vinci Code's prequel, Angels and Demons, has forged ahead despite the risk of a strike. The movie was already delayed by the WGA strike, so the producers were determined to get the film into production.
I think it's a bit risky to try and forge ahead with a production, even if there is a planned break if a strike occurs, much like what the fourth Terminator and second Transformers are planning. In fact, Transformers 2's crew will focus on visual effects and some second unit work (except stunts, since stuntmen are covered by SAG) if the strike happens.