According to a tweet from Google's head of Android Andy Rubin this morning, Android is seeing more than 500,000 activations per day.

If accurate, that is a significant number because it means that Android is gaining momentum heading into the slower summer buying season and later this year into the back-to-school and holiday busy seasons. According to Rubin, Android activations are growing 4.4% week-to-week. Can the little green robot keep up the pace to maintain its leading market share?

We reported from Google I/O in mid May that Google had activated over 100 million Android devices worldwide with 36 original equipment manufacturers producing 310 Android devices in 112 countries. At the time Google was activating 400,000 Android devices daily.

The tweet by Rubin is a significant occurrence in and of itself. Android's chief has a verified account with only five tweets, a low number perhaps considering it has over 29,000 followers and is included on over 1,700 lists. Rubin tweeted on Dec. 8 2010 that Android was activating 300,000 devices per day.

According to Apple's numbers announced at the World Wide Developers Conference earlier this month, the company has activated over 200 million devices (25 million iPads).

The It Portal, citing numbers from Apple, reports that the company had activating 366,000 iOS devices in Jan. 2011, which was before the Verizon iPhone was released. Apple was activating 230,000 devices per day in Sept. 2010.