1) Vine server is free.
2) This is apparantly something OS X handles natively singe Tiger (
http://macosx.com/forums/howto-faqs/52547-howto-simultaneous-user-environments-via-vnc.html). I'm pretty sure 10.4+ even has a built in VNC server, but if not there's plenty available for free.
3) You are correct that the best system would be an XWindows like client/server so that the user would run a client session where-ever they are and all you need to push over the network are input (keyboard/mouse) and output draw DIRECTIONS (not a whole screen image). That said,
4) If wishes were horses we'd all be eating steak.
5) This VNC login to a Fast User Switching account is something that does work, and I'm just looking for ways to improve it. Having realtime access to my GPU and half my VRAM to draw the screen for both users would be an improvement.
6) From the sounds of things, 10.5 or a late version of 10.4 fixed the keyboard input issues. Even so, it's not incredibly likely to cause problems with two people, in my opinion. Would I want to deploy this as an enterprise solution that way? No, but for two people at home it would certainly get the job done.
The scenerio is this: I have an iMac (see sig) that's plenty fast and is sitting idle most of the time, even when I'm actively using it. Wife has an old Dell machine. P4, integrated graphics, 512mb RAM, etc. It's fine for everything she does except she would like to be able to create home movies from our video and photos of our son. There's Windows solutions for this, but frankly, iLife is by far the best suite of applications for this. From iPhoto to iMovie to iDVD I think we can all agree that for even the novice user who is computer literate these apps are fantastic.
So, the choice is to buy her a new/used Mac ($250-500), plus whatever cost and time to find and learn some replacement software for a few things she uses regularly (MS Money in particular), OR we can hot-seat computer share my iMac so she can use iLife to do movie editing and photo slideshows and stuff on.
She's too cheap to buy a new computer (see 6 month old son referenced above) and while hot-seat switching would work most of the time, there will be times where the schedules will collide, and I like having MY Mac...
Now, if I can rig up her Dell to let her use my iMac from her seat it solves the problem for the cost of a $5 piece of cabling (we use wireless for our internet connection, so I'd create a little point to point network between our machines, likely using a length of FW400 cable, or a PCI gigabit ethernet card for her machine).
It would probably be fast enough to work alright, but if I could connect my mini-DVI port to her monitor (and her Dell over VGA) so she could launch the VNC app and then just switch monitor inputs it would probably so near seamless that she'd think she had a Mac.
Maybe I'm misinterpretting your post, but you seem almost hostle about it. I know it's not the BEST solution, but it is one that WORKS and works NOW. Hopefully Apple will dramatically improve the remote desktop and remote administration services, but in the meantime... well, getting this to work could easily save me over $500, so I think it's worth discussing.
Footnote: at work, using IE (blech), no spellchecker plugin installed, so I apologize for my sloppy spelling. I'm not stupid just extremely reliant on my spellcheck.