I'm running Windows 7 build 7057 (appears to be RC1) on my $299 Acer AsipreOne netbook from Costco and it works great! Win7 even found the built in Webcam! The latest install went just fine, booted off my USB key and did an upgrade from the earlier version of Windows 7 I was running (build 7000). Only problem was there were 4 devices listed as "Base System Device" in device manager on the machine -- they'd been unknown in build 7000 too.
In device manager these devices were listed as Vendor 197B and device 2381, 2382, 2383 and 2384. For those googling, this would be device IDs (as an aside, anyone know of a centralized list of these?):
A little hunting led me to an FTP site for JMicron's drivers, specifically ftp://driver.jmicron.com.tw where, within the /jmb38x/XP_Vista folder there are a list of drivers for the missing devices. Odd, since these are WHQL certified drivers, I would have thought that the centralized Microsoft repositoy in the sky would have found them. Maybe because they're certified for XP and Vista but not Win7?
I downloaded the JMB38X_WinDrv_R1.00.26_WHQL.zip driver and installed it. Worked like a champ! I now have four JMicron devices listed under the "Memory Technology Driver" branch on the device tree, and readyboost now seems to like my SD card too!
Added 15 Oct 09 I'm upgrading my netbook to the RTM version of Win7 and I see that JMicron now has version 34 of the drivers out, and they are Win7 certified now. Available at the same download location.