This does sound like two issues but from where I think the iChat one is it may be related.
Go to System Preferences > Security > Firewall and tell if it is set to Allow Specifics ?
If it it it should have a list of the Apps you have allowed.
This should include iChat and iChatAgent
(It seems that although you tell the system to allow it iChatAgent is not staying in the List).
Opps I see you have got to that part.
Ok this may be the issue that was caused by the 10.5 base Firewall settings.
Set it to Allow Specifics.
Remove everything from the List/
Set it to Allow All.
Make sure you have updated to 10.5.2. See below
Reset the Firewall to Allow Specifics.
If you are already at 10.5.2 and have NOT done the Time Machine and Airport v1 update then Rerun the 10.5.2 update via a direct
Combo download
If you have done the Time Machine and Airport v1 Update then download the Combo and
Pacifist
Open Pacifist (it will run as a trial)
Then open the combo .dmg
Drag the Brown/orange install .pkg to the Pacifist window
Use the Install option
(This Paragraph gets around an issue with the Time Machine and Airport v1 Update actually updating the OS past 10.5.2 But not changing the designation properly)
I hope this helps.
8:42 PM Wednesday; May 7, 2008