Restarting is the only way, I'm afraid. Otherwise, yes -- you'd need Parallels or VMware Fusion to do the trick. Using these products won't run Windows at full speed though, so Boot Camp is probably your best bet.
When Leopard was first announced, there was a tidbit on Apple's site that said Boot Camp was going to have a fast switch mode, where you could just put OS X into Safe Sleep mode, and have Windows put into Hibernation whenever you switch operating systems. Would have been a lot nicer, but didn't make the cut... as seen
here.