Google has announced on the Official Gmail Blog that they have begun rolling out default https settings to all Gmail accounts. For users that don't want this setting, or if it causes some sort of connectivity issues through someone's firewall, the feature can always be disabled. Here's how it will work...
"If you've previously set your own https preference from Gmail Settings, nothing will change for your account. If you trust the security of your network and don't want default https turned on for performance reasons, you can turn it off at any time by choosing 'Don't always use https' from the Settings menu."
Google is doing this to protect people's private data from being sniffed by third parties. If someone is using a public Internet connection, they are at risk having their traffic monitored. The reason this was not enabled by default, up until now, is that https slows down data transmission. This is due to the fact that the information must be constantly encrypted and decrypted. Google, however, after much research into the issue, feels that https doesn't really cause any significant slowdown from a user's standpoint. They concluded that the security benefits outweigh the slight performance hit that some users may notice.
Google notes that the enabling of https may cause problems with offline Gmail, if you currently use the feature over http. As well, many commenters have pointed out that the iGoogle email widget is not compatible with https.