Entourage 2008 does not contain "format=flowed". The item keeps getting bumped by things Microsoft considers higher priority (more people asking for it). Some users (but apparently not enough of us) have been clamoring for this addition for years, and it is definitely on MacBU's radar...so possibly, if you and whoever else who reads this and wants this capability would use the Help menu's "Send feedback" option to request it directly from Microsoft, we might generate enough noise to raise its priority.
"format=flowed" basically, is unwrapped text without automatic line endings, something many other mail clients have.

Comments (3)
I will send feedback again.
I sent feedback too. This is ridiculous: entourage 2008 breaks an 82-byte URL into 2 lines, whether or not I wrap it in angle brackets first. The result can be clicked (with or without brackets) but only the first 80 bytes show up in the browser. Guess what? partial URLs don't work very well.
When I received the email notification for your comment it was 3 lines and it works. Not sure what is going on in your case. I'm replying to you privately so we can test some URLs.
In the mean time, you can use Tiny URL to create short URLs.