So I was just contacted by Brett Smith from the FSF about licensing issues for server-side software:
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Yes: the Affero General Public License. This is a modified version of
the GPL, with our blessing, that simply adds a requirement exactly like
you describe. We encourage you to use it: the current draft of GPLv3
will be compatible with the AGPL, so once GPLv3 is released people will
be able to use your code in other projects released under GPLv3.
You can find the license, and more information about it, at
http://www.affero.org/oagpl.htm l. The new requirement is in section
2(d). If you have questions about it, please let us know.
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I'm thinking about switching to this license.