Right now the PDF exploit that jailbreakme.com is using to jailbreak your device is a HUGE security issue. Right now you could brows to a website set up by an attacker that would install a virus onto your device the same way jailbreakme.com can execute their own code and jailbreak your device. You should open Cydia as soon as possible and install the package named "PDF Loading Warner"
After installing this patch, when ever a website tries to open a PDF file you will get a warning message giving you the option to continue and open the pdf or cancle. After installing this patch go back to jailbreakme.com and test that the patch is working by sliding the jailbreak me bar as if you were going to jailbreak the device. Instead of the jailbreak starting you should get a popup that looks like this.
This doesnt completely block the security hole, it just prompts you when a PDF is loading in safari so that a website cant just load one without you knowing. If you have not jailbroken your device, now might be a good time to do it. Even if it just to apply this patch so that your device is not vulnerable to websites that may try to attack it. Apple should put out an updated iOS real soon to fix this. within the next few days, but until then be careful with the websites you visit if you don't apply this patch.