This is a problem with several DHTML editors -- notably vBulletin's -- which don't work quite right under mobile versions of WebKit. (MobileSafari, Android Browser, WebOS Browser... all different branches of the same web browser source.)
You can tap-and-hold or double-finger-scroll to try and scroll within the editbox, but it doesn't always work on vBulletin for some reason. (Which is annoying, since it works on Invision and phpBB 100% of the time; it's apparently something in vBulletin's javascript.) As noted, a Bluetooth keyboard is about the only guaranteed-effective way around it for an iOS device. (On an Android device, you have the option of using the d-pad or scrollball/optical selector. Or the cursor keys on a Palm Pre/Pixi. iOS has no physical cursor keys unless you connect an external keyboard, however.)
vBulletin does have a third-party option called 'ForumRunner' to make boards more accessible to iPad users -- it's a module you install on the bboard that lets the content be read entirely as XML, and then displayed in a 'ForumRunner' mobile application using native controls rather than webpages. I'm not sure the admins would be amenable to installing that here, though, since I don't know how it works with password-protected stuff like the SYW forums.