A year and a half ago, Matt Cutts of Google released a video titled Can switching to HTTPS harm ranking?
In that video he explains that technically switching from http to https for all your URLs should theoretically be fine but that not many sites have done it. So if you plan on making the switch to HTTPS, then make sure to test it slowly on smaller sites.
Now he is more for sites going HTTPS with less of a worry about how Google may index and rank the site.
Matt said, "at this point I'd say feel free to go HTTPS."
He does offer some caution saying:
As always, I'd test it out first, ideally with a lower-traffic domain name. But if you like the results you get, you should be able to use our existing tools like 301s and rel=canonical to help Google discover that the site has gone HTTPS.
Here is the video from a year and a half ago:
Forum discussion at Hacker News.