Steve, you have several options....
1. Shave your head(If you can pull it off well)
2. Get a hair piece/system
3. Use topical concealers(lookup Dermmatch/Toppik(hair fibres)
4. Try medications(Rogaine/Propecia)
5. Get a hair transplant
I have made use of topical concealers to very good effect for about 1 1/2 years...If you have diffuse thinning, you can use dermmatch to basically paint your scalp, and then use toppik to sprinkle hair fibres overtop to simulate real hair. This can give you a full looking head of hair again which depending on your degree of loss may not be detectable, or only detectable under intense lighting.
I recently went and got a hair transplant myself in December 2006 of 5800 follicular grafts(1 graft = 1-3 hairs usually). It was taken out as a strip from the back of the head(donor area) 30cm strip by 2.5cm thick(scar won't be visible once healed unless shaving lower than a #3 or #2 guard)...then dissected by a group of techs...The doctor then makes incisions in your scalp for the techs to place the grafts back into the balding areas.
The average person has about 8000 grafts to harvest depending on donor scalp laxity and hair density(In several sessions, it's rare people harvest over 5000 grafts in one session). It takes about 3 months before the transplanted hair begins to grow out, by month 6-8 you will see 90% of the results, and in 1 years time your transplanted hair will be fully thickened and of the same texture as the surrounding native hair.
You can obtain a full or full-looking head of hair again depending upon your degree of hairloss and size of your procedure...The average human head has 100 grafts/cm2...in order to obtain the illusion of a full head of hair, you only need to transplant at 50 grafts/cm2. Once mine is fully grown out, I will obtain a full looking head of hair, I'll just have a bit of a thin crown which I can always get fixed in a later surgery(I'm a Norwood 5, somewhat advanced).
The whole thing cost me 20k(I got a big procedure though, might cost you around 12-15k)...If you're going to do surgery, picking the top doctor is key. The best in the industry are pretty much all in Canada..I would suggest either Hasson&Wong(Vancouver) for conservative hair placement or Dr. Armani(Toronto) for a more aggresive youthful hairline.
There are many actors who have had hair transplants...Mel Gibson, Jamie Fox, Matthew Mcconaughey to name a few..
Then there are those who wear hair systems like Ben Affleck, Brendon Fraser and John Travolta...
(There are pictures which clearly show this which you can find on the internet if you search around enough)
If you consider a hair transplant, do A LOT of researching first on internet forums...such as this one:
http://www.hairlosshelp.com/forums/