Muqq and izzolite have argued that there is no "haste cap". There is however, in my opinion, a point of intersection. This intersection is the point where Spell casting speed out paces player skill. This intersection is not simply a static point that is applied to all players, but instead a sliding scale whereby players of varying skill will reach this intersection at varying points. Remember, as you increase your haste rating three things happen. Your Damage over Time spells and Mind blast have a shorter duration/cast time, your channeled ability (Mindflay) becomes shorter, and your Global CoolDown (or GCD) lowers. As each of these three conditions change, you, as the player, must also change. You need to react faster, being more aware of your timers, and be ready to recast abilities sooner, and sooner. For some players, this may not happen until they reach the GCD thresh hold (the point at which spell casters can no longer reduce their GCD...this), which is 1.0 seconds. Some players will reach this point well before that. This is something that only the individual player can determine, and is something that should be constantly tested.
There is also the matter of dps downtime as a result of excess haste. As I said before, and has been stated for some time, the GCD cap for a spell caster is 1.0 seconds. This is a problem that is more or less exclusive to Shadow priests and Balance Druids.
Vampiric Touch, and Mind Blast have a base ( or non-haste modified) cast time of 1.5 seconds. Your GCD is baseline (non-haste modified) at 1.5 seconds. As you increase your haste through items and gems, your VT cast time, and duration begine to decrease, this applies also to Mind Blast. Your GCD also begins decrease. The problem at high haste/gear levels lies in the ability for spells, and not the GCD to continue to cast faster even when the GCD can no longer be hastened. At this point, you will find yourself with dead time. This dead time, over the course of a fight, can be rather detrimental to your performance. There is nothing that can be done about this. Once you reach 1.0 second GCD, you are NO LONGER ABLE to increase your cast frequency. As many Shadow priests who use Engineering are beginning to learn, once you reach around 1200 haste (this is not a far fetched number.. in less than 50% heroic gear I all ready have 1204 haste), activating hyper-speed accelerators brings your VT/MB to around .90-.83 seconds...dependent on the actual level of haste from gear/gems you all ready have. Consider that Hyper speed accelerators provide 340 haste (or approx 10.5%). Bloodlust provides 30% as a comparison. The question I have been trying to flesh out is whether the benefits to DoT's outweighs the loss of cast activity caused by going under GCD cap every 60 seconds + the 45 seconds of Bloodlust.
Edit: As "the not so evil" pointed out in the post below, Lag will play staggering role in how haste affects your dps gains from haste. This is something that can not be prevented, and will always have a detrimental affect. While it is possible to mitigate minor latency changes, such with cast bar mods like Quartz+MF2 which can provide a buffer zone (the red section in the cast bar that changes) for spell queuing and MF ticks. At the end of the day, if you are player who consistently has lag spikes, or generally high latency, your "intersection" is most likely to be markedly lower than a player of equivalent gear/skill with a lower latency. When testing your haste value in this fashion, always consider things in your game environment that will directly impact your ability to smoothly and quickly cast spells. The ability to maintain a high "activity" percentage is a cornerstone of good dpsing.
While your statements here are fairly true, I think you're looking at this in a bit different then Muqq/Izo/etc. Most of the discussion that occurs on this topic is mostly to do with optimal playing. In every theorycrafting scenario haste will improve your DPS even over the 1640 mark. The main problem/discussion point past that concerns MF Vs. MB and generally depends on the player at this point.
Regarding cooldowns, there is a point where just outright popping everything is going to hurt you more then help you, you are right about that. The common theme with said cooldowns is to stagger them out. You and I are trolls for example, on a fight like Festergut we could stagger our Haste CDs to Bloodlust -> Berserking -> EngiGloves -> Potion. The only wildcard there would be Black Magic (which even if it procs during other cooldowns, it would still technically be a DPS increase).
I do agree with the basic point that the player makes the gear, and the player is going to have to adjust himself accordingly. However, given numbers alone Haste (as it is right now) is always going to be a DPS increase stat versus other possible choices.