The installation guideline is very insufficient. I've installed for Debian lenny/sid with the following step;
apt-get install php5-dev (to enable pecl command)
apt-get install libcurl3 (to get curl support)
apt-get install libmagic
pecl install pecl_http
pecl will download and compile but before that just follow the prompt. If the respective development library is not available in your system, the compilation will abort.
After pecl_http got install, copy /usr/lib/php5/20060613+lfs/http.so to /usr/lib/apache2/modules and restart apache
Hopefully this will help others from facing the same hassle for quite sometime.