much less the delayed 'snap' of the layout into place as the CSS gets delayed waiting for the Font
Depends on the browser, unless Cufon specifically throws in a default font until the other font can get loaded. (wait, I have DejaVu Sans Condensed... wow, OP's giving a Linux font to Winblows users?? hahahahha that's awesome)
Some browsers show no text until they have a font, while others show a system default until they have the font.
With jquery I just find IE7 is the lowest one should go really. IE5.5 and IE6 is slow anyway, visibly slower than IE7 which is by modern standards slow anyway.
We need to move forwards, evolve, modern pages need more processing prowess.
Careful there. If your site is an intense web application for high-powered rich folks and pampered gamer kids sitting with expensive equipment and ADSL in their parents' basements, yeah, you'll drop IE6 and make a bunch of assumptions of your target group to give them the experience you want to give them.
But MOST websites are just websites. It's true, developers do what childless couples do in McMansions: we fill the space without thinking. I get a new fast fancy computer with phat-cable super-snel download capabilities, I start assuming everyone else does internet that way except maybe grandpa with his Windows 98. But it's not so.
I have a poor view of jQuery plugins as most of the ones I've had to touch either needed extensive help just to get accessible (do jQuery people know what a keyboard is?? .slider for example) or someone else has modified greatly to make it accessible (which I like much better because I'm lazy... example <--they even have ARIA junk in there!). Make sure your site works with plain, valid markup, does all the stuff it has to do, and test in all browsers and other user agents (mobile users don't haz teh mice, etc).
Then layer the java-junk, funkmaster fonts and the 1terabyte gradient images or whatever on top of that. A site that looks really cool but takes more than 10 seconds to load is a site I've probably left before it finished. Users do that. Speed matters. The higher someone's connection, the less time they are willing to wait... though this also means those with crappy connections are willing to wait longer because they always do.
As a developer you are allowed to kiss IE6 users goodbye. It's your site. Some people even forget IE altogether because they are aiming for web developers, who tend to use slightly more awesome browsers. But restrain yourself from requiring everyone has dual-core processing, lawlz.
The Cufont effect is granted a delay on page init but in modern browsers the delay is 1-2 seconds, providing it's run on a modern computer (i.e. Core 2 Duo).
What do you want for people using anything other than a modern desktop browser? Do people use phones in your country? (In my country they are used all the time for Internet, but I noticed much much less of this when I was last in the US for instance)