anthias - some more information about your environment might help? Judging by your error your on 2.4. Have you got any modules installed (what versions) or lots of custom code?. Also post the full error message / backtrace.
This same problem just hit 3 of my client sites running SS 2.3.x on PHP 5.2.17 (in my case). I did a phpinfo() check and found a module called "http" that supplies a number of built-in classes, one of which is called HttpRequest which matches the server error. I think the change to SS_HTTPRequest (and SS_HTTPResponse) needs to be backported to SS 2.3.x or else I imagine this problem is going to crop up all over the place.
The "http" module lists the following under its "Registered Classes":
Confirmed with the host. They installed the PECL HTTP extension which is what is causing this problem. Since PHP classes are case-insensitive, the HTTPRequest class that is being instantiated in Director::direct() is the PECL version (called HttpRequest) since that class is pre-loaded into the PHP environment rather than using SilverStripe's __autoload() handler to load the SS version. Waiting to hear from the host for a solution. Hopefully won't have to do an emergency SS upgrade to 2.4.x.