UPDATE:As of April 14th this workaround is no longer functional!

There's an easy way to keep the OtherOS functionality and still have access to PSN. Just trick the update manager to think the latest firmware is already installed. This can be done by a simple DNS redirection.

All you need is access to a DNS server and a web server. I got a hacked Buffalo Terastation Live with Bind and Apache installed.

Get the EU or US update list. Modify it to have all zeros as version numbers:

# EU
Dest=85;CompatibleSystemSoftwareVersion=0.0000-;
Dest=85;ImageVersion=00000000;SystemSoftwareVersion=0.0000;CDN=http://deu01.ps3.update.playstation.net/update/ps3/image/eu/2010_0401_3a08ef6164a7770ae3e7d5b9f366437a/PS3UPDAT.PUP;CDN_Timeout=30;

Place the file into a /update/ps3/list/eu/ (for EU) or /update/ps3/list/us/ (for US) web directory. If you are using virtual hosts, handle feu01.ps3.update.playstation.net or fus01.ps3.update.playstation.net in your web server configuration. Then add the corresponding DNS zone, configure the PS3 to use your modified DNS server and you are done. PSN no longer cares about your PS3's firmware version.