It didn't take too long for the talented M33 team to release a custom version of Sony's latest Skype-enabled firmware. With the new Network Update feature of these custom releases, homebrew users can simply update directly from their systems. However, those that aren't fortunate enough to be near a wi-fi point can simply download the necessary files here. (But, considering Skype is the main benefit of 3.90, we think only those with reliable access to wireless internet will really download this update.)
Posts with tag 3.90
Firmware 3.90 fixes mysterious Store errors

Supposedly, the fix only affects PSP-2000 systems, but it's clear that each firmware update Sony releases fixes a few more issues that users are experiencing. Hopefully, PSP Fanboy readers afflicted with this mysterious glitch have found their woes alleviated.
[Thanks, Hashbrown_Hunter!]
Firmware 3.90 now available on Network Update

- Support for PLAYSTATION Network titles has been expanded.
- Skype has been added as a feature under Network.* (Only for PSP-2000 series.)
[Thanks, Jorge H.!]