I expect to have 1000+ subsites in a site-collection and would like to setup a quota for the subsites, is that possible in SharePoint 2007 & how ?
You have to decide the IA ( information Architecture ) before the start of provisioning, ....This will help to decide site structuring DEEP vs WIDE
DEEP Structure : having one sitecollection to house all the sites requests as subsites
-> cannot limit the site quotas -> security nightmare (1000+ sites) -> as the content DB grows , it would be difficult to backup, migrate and restore
WIDE Structure : Each request a seperate site collections
-> Controlled Quote Sizes -> manageable security -> Isolated and easy to backup and restore -> room to grow
1000+ subsites is a nightmare to manage
Read this article.
Plan ahead and manage easily(wide).. OR no plan (DEEP), cab be controlled by constant monitoring :)
As Wictor say quota can only be set on site collection -no workaround.
An alternative is to monitor site sizes using Storage Management
but be aware that this functionality has been removed in SharePoint 2010 since it caused performance issues for end users.
There is a third party utility that can help you manage this, try the SpeakTech governance kit which does much as well as allow for Subsite Quotas, be advised it is expensive though.