How do I let some certain vertices stay still when sculpting?

For example, fix the border vertices when sculpting a open surface.

And BTW, is there any hotkey for entering sculpt mode?

Nicholas Bishop is working on porting a 2.49b very usefull feature that is not yet in 2.6x :

It worked great , not only to improve performance a very lot, but to work around the lack of mask function too, as what wasn’t displayed wasn’t touched at all by the brushes.

But 2011 sculpt gsoc had a mask function, Nicholas Bishop recieved some funding to port some of the GSOC sculpt improvement into official Blender, maybe he’ll give a go to that gsoc mask function too.