Warranty coverage gets denied if you present your mac for service and it's obvious that damage has been caused that wasn't made in the factory. Adding anything that isn't user replaceable (according to the user manual from Apple) means special tools, special training, and/or limited availability of proper replacement parts. Not being static safe can cause damage to other components, so it's not so much "can you install the hard drive" and more "will you not damage anything else while you're in there".
You'll want to ask the seller if the replacement drive has equivalent temperature sensors and firmware to work with your mac and what their return policy is if you find your mac won't sleep or overheats with a replacement drive added. The risks are very low for DIY repairs if you take your time and can work on things small as eyeglasses screws. But the cost of failure could be a major repair - logic boards are often in the $600 to $800 range. Once you have a unibody back off, that part is exposed. Use care to not disturb the battery - a loose screw between the battery and case is a serious fire hazard.
I find the manuals at ifixit.com to be a great place to see how involved a repair is so I know if it's worth investing in the tools and time to attempt a repair.
Armed with more information, you can decide how much your time is worth and whether you want to pay a professional.
The pros will let you know to bring the mac back so they can put in the original HD for warranty service and they will ensure their work is up to Apple's standards so you don't lose warranty protection.
DIY is an awesome feeling and less expensive when things go well.
As to specifics, there are some great mid price level SSD around and the kits for mounting a SATA drive in place of the optical drive from MCE and OWC look to be equally fabulous items. The new SSD that run at 6Gbps are starting to ship so the "old" models are starting to drop in price. I plan to take advantage of this price drop and grab an SSD myself in the June time frame. Adding about any SSD will add significant snap and more useful life to any 4 year old or less MacBook Pro.