Depends when Nokia entered the market.
If they'd entered quickly at the time when they decided to go with Microsoft they could easily be a major player in the Android market.
Some of their hardware is good and they were one of the biggest brands in Mobile phones in Europe (pretty much everyone had a Nokia phone at some point).
In the early days of Android HTC were the leaders by far in the UK. In fact very often people would call an Android phone "an HTC", as in "are you getting an HTC?". Android wasn't ever marketed by the stores or the Manufacturers so you didn't readily know what was running on the phones unless you were a bit tech savvy.
However, HTC seemed to get blasé about their popularity with the G1, G2, Hero and early Desire models being early leaders. However, a lack of support, bugs, lack of updates etc, IMO started their decline.
Samsung capitalised, made some bold and brave choices and has become the runaway success it is now.
I hope HTC can return to their former glory and capture a larger part of the market again as they are pioneers in the smartphone arena, way before the iPhone was a twinkle in Jobs' eye.
Then again it would also be good for Sony to get some recognition for some of the great devices they produce as well.
If Nokia entered the Android race now, I easily think they could get more sales than their Windows devices, quite quickly and they could also innovate enough to take a significant chunk of the Android market.