I would highly advise they remain free, in the catchup position they're in, having updates that you have to pay for is a very very bad idea. As it is I hate capitalism, if it were confirmed that even one update were to be paid for I would immediately jump ship back to Android.
if you watch this video presentation of the Multi-tasking at the end he says all the new features he showed will be coming in a free update.
I'm sure Multi-tasking is not in the march update so can only mean the big update later is free too YouTube - WP7 Multitasking Caught on Video
If MS began charging for their WP updates, of course it would piss everyone off.
The other thing that would happen, is that all the people with Windows Phones that don't really keep up with the details would miss out. They may get a notification for an update, but if it costs any money at all (or even if a free update is not easily implemented) they will not do it. This will mean that a good portion of the consumers will not get full use out of their phones. This would include features that MS has spent considerable time and money developing.
Microsoft certainly knows better than this. They would be throwing away a lot of their hard work just to make a little money. They will make so much more if they can get the OS working better.