Quote - You never know where a  'hobby'  will lead.  If you get hooked on digital imaging like I did, you will be glad you spent your time learning the application most pros use - Photoshop.    In addition, you're plugging into Adobe, which means easier crossover with applications like 
In-Design, ImageReady and now Dreamweaver.  

My interest in digital imaging started as a hobby 4 years ago, and now I'm looking to do it as a living.   I've never regretted learning Photoshop, and at times, give a sigh of relief I chose it.   There is so much professional community support on the web, in books, in 3rd party products, in Adobe support.  

It seems to me investing your time learning Photoshop gives you far more options should your 'hobby' turn into a passion.

I'm one of those who turned a hobby into a living, too, so this is good advice for those who follow that path. BUT, like I said, many end up not following that path -- either because it's just not in them or they find Photoshop too intimidating -- and no one can really tell early on where their path will take them. So it's not good advice for everyone. Spending big bucks on PS at this early a point can end up being wasteful, no matter how many options it gives you ... because if you don't take those options, what then? You've spent $700 instead of $70.

If you DO want to learn PS because you think you WILL follow that path, do as Gongyla suggested and get Photoshop Elements first -- it's cheaper, and it actually comes free with a pen and tablet set.

I'm a HUGE fan of Photoshop and consider myself advanced in it (see my tutorials), but when I started doing graphics as a hobby, I started on Paint Shop Pro 5 and worked my way through Photoshop LE (which came free with my Wacom) and, later, older versions of Photoshop itself (4, 5, 6, and 7 at work), until finally, I got myself the latest Creative Suite 2.3 for my Mac. And what do I do now? I do book covers for several publishers -- for both print and web. I do magazine ads, catalogs, business cards, and the like. I DO make a living at it, and my point is, I didn't have to spend a lot of money in the beginning to get here.

In fact, if you're resourceful, you can broaden your skills without investing a whole lot of cash. Then, when you're ready to go pro and your skills have outgrown your tools, you can consider increasing your software budget and investing in something relatively expensive, but solid like Photoshop.

Just as an addendum, here are other ways (all legal, I promise) to broaden your skills and get free/cheap software on which to learn BEFORE you spend a lot of money:

1. Free tutorials on the web. Google is your friend.
2. Online digital art communities, like Renderosity and the like. The forums and your fellow helpful members are your friends.
3. Digital art magazines, like 3D World and Computer Arts. The free CDs that come with them have software you can learn before you decide to buy them, like Bryce, Poser, Vue, etc., and they are VERY much your friend; all told, you'd be paying $13 to have access to software that would normally cost you $300.
4. 30-day demo software.
5. Wacom pen and tablet sets, if you're into painting at all. They come with free software like Photoshop Elements and Painter Classic.
6. Student discounts, if you're a student.
7. Friends with old versions of graphics software they no longer use.
8. eBay.
9. Work. If you're anything like me, you might be able to find a day job that gives you the opportunity to play with the software you want to learn.
10. Paying clients. When people start paying you money for your work done in Paint Shop Pro, you can put that money aside to finally buy Photoshop or whatever else you want to get.
11. Explore, experiment, and practice! Always.