Sometime last January-February, after the Nexus One came out, there was talk and rumors of its next successor. The talk was it was Motorola's turn, and that this phone would have a sliding keyboard. Then as months rolled by, the Nexus One wasn't burning sales numbers, and I suspect, Verizon and Motorola put some heat on Google, and the Nexus project was pulled back.
I believe the original Nexus Two was the phone that ended up being the Droid 2, except with the bare Android 2.2 Froyo and no Motoblur. My hypothesis was that this phone was supposed to have came out around June, around the time Froyo was released. Usually, the pattern is, everytime there is a major Android release, there is some Google phone leading the way. If it's not a Google phone, then something that had Google input into it. Before the Nexus One, it was the G1, HTC Magic (Android 1.5) and Motorola Droid/Milestone (Android 2.0). Nexus Two went back to the boards, revised with Motoblur and came out as the Droid 2.
So there, Froyo came out without a customary flagship.
Then again, another phone, also with a sliding keyboard, might have been meant as the Nexus 2 before it was aborted, then renamed and re-marketed. I believe that phone ended up as the HTC G2.
So skip to Gingerbread now. Despite "canceling" the Nexus project, Google will always need some ADP (Android Developers Phone) one way or another and that surely will always be around as long as Android is around. So who will be the next ADP? Surely the Nexus One is aging and its not a perfect design.
I would have assumed it would use the MyTouch 4G as the basis. Take the same phone, no 4G, no HTC Sense, then reorder the buttons to the way Google likes it (<= on the left). That for me would have been a logical next Nexus. Another reason for an HTC phone is because I've always assumed that Qualcomm is the "favored" processor Android and the current Qualcomm flagship is the Snapdragon 8255 found in the MT4G and the Desire HD.
So basically, Google going Nexus on Samsung and Hummingbird is a complete surprise. But i can see the reasons. As it stands now, the Galaxy S is obviously the most successful Android phone to date. Developers should should factor the Hummingbird in their game development (devs tend to concentrate on the Qualcomm Snapdragon first). If you really need to plant your flag on the hill it has to be the biggest hill. Samsung also has some aces where HTC And Motorola doesn't have much experience in doing, such as using NFC (Bada Wave phones have them) and frontal cameras (international Galaxy S has them, among other Samsung phones).