I didn't see a topic like this on the PS3 board, and while it may be old news to some I intend to share with those not in the know and possibly provide new information about becoming a werewolf/vampire hybrid.
First it is possible. But before sharing how to become one it is better that I explain the three paths one can follow because there are two different hybrids to consider, the stage 4 hybrid and the feeding hybrid.
Stage 4 Hybrid This hybrid is stuck as a stage 4 Vampire and cannot feed. Townsfolk are not hostile towards you but city guards are (based on a majority of forum posts). This has the benefit of not needing to feed, allowing you to fast travel without worsening your condition, and access to all vampire powers. The game is mostly playable (note that some missions such as those where you speak to guards or certain NPCs may require you to cast calm/pacify on them to talk to them. The major downside is that you need invisibility and muffle to get around cities without constant assault from guards. Entering strongholds is very dangerous due to the high number of guards (most notably in Riften).
Feeding Hybrid This is a typical vampire; you can feed and progress though the normal vampire stages. As such it has all the normal pros and cons of a normal vampire. It is not known if progression to stage 4 vampirism will convert you to the stage 4 hybrid described above.
How to Become a Hybrid You will need to do the quest "Ill met by moonlight" which is given to you by a prisoner in the Falkreath Barracks. Note that in this quest you need to obtain the uncursed Ring of Hircine by not killing the Werewolf (it is possible to also get the saviours hide with he ring see: http://www.uesp.net/wiki/Skyrim:Ill_Met_By_Moonlight). You will not need to become a werewolf through the Companions questline and obtain an extra hagraven head for yourself during the mission "Blood's Honor". Finish the Companions questline and equip the Ring of Hircine and the Power Ring of Hircine and save the game. Cure yourself of lycanthropy and kill your spirit wolf. Confirm that you can sill become a werewolf using the power Ring of Hircine and that you do not have beast blood. Now contract Vampirism from a vampire's vampiric drain with the ring and power still equipped. This is where you can choose which type of hybrid you will become.
For stage 4 Hybrid Wait the required number of days for you to become a stage 4 vampire. Transform into a werewolf away from NPCs and allow it to wear off. Confirm that guards are the only ones hostile towards you.
For Feeding Hybrid Wait the required number of days to become a stage 1 vampire and make sure you feed. You should remain a normal vampire as long as you do this before stage 4.
Curing hybridism (unconfirmed speculation) Perhaps you want to change what type of hybrid you are, or do not wish to be either a vampire or werewolf anymore. A stage 4 hybrid is rumoured to be cured by contracting the vampire disease again and feeding then curing vampirism through the normal method. A Feeding hybrid should be able to just cure vampirism like normal.
Note: Through this method it is possible to become a werewolf/normal hybrid as well by not contracting vampirism. This is useful if you do not want NPCs commenting about you smelling like a dog or having hair in your ears but is of very little use beyond that (unconfirmed: you may be able to get restful sleep bonuses).
You can also become a hybrid without the Ring. You need to contract Sanguinare Vampiris before the Companion's quest where you drink the blood (The Silver Hand), go near the pool of blood with the disease. Wait exactly 72 hours (this includes the time it took in between getting the disease and getting to the Underforge) then drink the blood just as you contract Vampyrism ("Your blood boils").
Guards will get aggro at Stage 4 though. If you're level 60+ as I am, this forces you to get the Animage+Kindred Mage perk and Harmony to calm down the guards. On top of guards, these will also attack:
* Louis Letrush * Vampires (eg: Alva)
Because guards attack unprovoked, friendly NPC's will rush to your aid in fighting the guards.. and usually tend to die doing so.
OoO I had no idea you could pull this off 8D I may just have to do this on my battlemage Dark Elf >.> I really wanted to try being a vampire (even tho I love being a werewolf more), and it looks like I can :3 Thanks!