I think Ubuntu 8.04 is ready for anyone who wants a good desktop computer with a decent set of office productivity tools. The installation and update experience with Ubuntu is as good or better than Windows and Open Office does what the vast majority of ordinary users need it to do. My mother is using Ubuntu for e-mail and web surfing and having no problems (beyond what she'd have with Windows seeing as how she's never used a computer before).
Where I'm getting blocked is when I want to do the power-user type things: e.g. sync to a PDA that only has Windows Mobile 2003 on it or use cutting edge devices like a Lenovo x300. That's when I find I have to go to Linux Geek Land. At least I'm only recompiling modules, and not the whole kernel :-)
So in that sense, Linux is ready for the average person's desktop, it's just not ready for us geeks yet.