So MMS and Facetime may have broken for you after the jailbreak (it did for me) – although I’m confident that a fix is on the way, there’s a quicker way for the impatient among you. (Thanks to @_Appman_)
Go into Cydia, go to Manage > Sources > Edit > Add and type: http://iphonedelivery.advinux.fr/cydia
Next, download “iPhone Delivery” package from that source. Respring when prompted and check your Messages app. MMS should be back. Next, go to Settings > Phone and turn Facetime to “ON” and wait for activation.
I now have MMS and Facetime back on my jailbroken iPhone 4. Hope it works for you. Thanks again to @_AppMan_ for the tip.
EDIT: Actually there’s an automatic update now in place thanks to Saurik and Comex. Simply open up Cydia after you’ve jailbroken and install the mandatory update! Done. Easy..