Typically, you need to worry about companies that promise a top position in Google's organic or free listings. But Google also warns that you should be wary of those companies that will guarantee the top position in their paid listings, Google AdWords.
Googler, AdWordsPro, said in a Google Groups thread:
Bottom line, if anyone says they can 'guarantee' top placement - whether in the unpaid search results or within AdWords - then buyer beware.AdWordsPro
Why? Well, Google's AdWords algorithm is not simply based on how much you pay. Other factors include your click through rate, the quality of your pages and many more criteria. This also now applies to Yahoo Search Marketing and Microsoft adCenter sponsored listings.
Forum discussion at Google Groups.

Comments:
Nagaraju
11/19/2007 12:39 pm
Hi, Most of the companies were offering the top positions for the keywords in the Google. but i can't believe that one because i have paid some amount to one SEO company to get good results in google.
Lyon
11/28/2007 03:02 pm
personally Google is the highest for searching and ROI for us but their organic listings are not very fair in regards to placement anyhow! if there is a loophole, i do not blame anyone for utilizing it to their advantage. However, i thought reciprocating links had a big role on placement or am i wrong.
No Name
07/22/2009 09:46 pm
These guys are failing at their jobs by not staying on top of internet marketing technology, top placement has been around on Google and Yahoo for 6 years.....all the seo guys never say its possible becuase it puts their service to shame