encouraging image caching with ExpiresByType

do modern browers even obey this at all?

[amznVibe]

I can't seem to find a discussion here at all on the ExpiresByType command for Apache

In the ever important struggle to reduce bandwidth (and decrease page load times)
I discovered these commands elsewhere as a suggested inclusion for .HTACCESS

ExpiresActive on
ExpiresByType image/gif "access plus 30 days"
ExpiresByType image/jpeg "access plus 30 days"

Are these really effective at all? Since 75%+ of visitors use IE 5.5 and IE 6.0 I can focus my question on those browsers.
Will IE even obey such a suggestion to store images that long? Or will it use a "least used first discarded" algorithm anyway?

I'm gonna do some more research but I figured I'd bounce this question off the more savvy memberbase here.

[BjarneDM]

Every browser has a set amount of memory avilable.
When this gets full, the least used files get deleted and replaced.
Setting the expiered time only works for something that's *in* the cache.
You can't force any browser to store anything for any length of time.
The expired time *only* determines wheter the browser bothers to check with the server or simply by default uses an excisting copy in the cache.