"This key may be unsafe"

[Charly Avital]

GPG Keychain Access 0.8.4 shows a red warning 'This key maybe unsafe'
for *any* key with a length equal or inferior to 1024 bits.

GPG Keychain Access 0.8.4 is a GUI for key management for Mac users.
<http://www.gpgtools.org/keychain.html>

A Google search with key sentence "This key maybe unsafe" between
inverted commas, to limit the search to the whole sentence, displays
hits that relate directly or indirectly (Twitter) only to GPGTools' lists.

I am cross-posting to gnupg-users to try and get more feedback about
this issue:
Are keys whose length is equal or inferior to 1024 bits *unsafe*?
If so, how are they unsafe?
Where is this key length unsafe situation documented?

As a personal example, my primary key A57A8EFA is a DSA "old" 1024 bit
key, but its encryption subkey is 2048 bit long, and I use a sign-only
2048 bit long RSA subkey. I also get that red warning with GPG Keychain
Access 0.8.4

TIA.
Charly