Installed to my T42 today (won't have the machine much longer, but thought I might as well test RC4 on it while I can).
I tried the 'verify installation files' button again. Still nothing useful, but this time it didn't kill the install, as it did on the T61. However, at the end of the install, I got the network configuration screen for a second, then the installer died. While I managed to restart the installer, it was right back to the beginning, which wasn't very useful. Did a clean boot and tried again, this time without touching the 'verify install files'. That install went through without problems.
Working fine on the T42. On interesting tihng - the update notifier was active from the first boot, even though the service was not marked as active in VasmCC.
Other than that, everything seems to be runing fine. I get better wifi signal levels on the T61 than I do on the T42, but whether that is a driver issue or simply that the T61 has a better wireless card, I don't know.