Choosing an SEO Service

Here's a few pointers that may make choosing your SEO services provider a bit easier.

Picking an SEO service based on search engine rankings can be deceptive, the question as to whether they can replicate their own success on your behalf depends largely on how they got their rankings.

Here's what I'd look at:

Their Linkage

By their links shall ye know them, put their Url into Yahoo Site Explorer and take a look at their inlinks.

If most links are coming from client or other "controlled" sites they're unlikely to be able to replicate their own success on a clients' behalf, if most of their links are coming from blogs/forums that at least makes it in theory easier to replicate.

Ideally no single source of links should dominate.

Clients' Linkage

Worth looking at the linkage of client sites to check for link commonality such as interlinking, creating a link network between an SEO and their clients can have unpleasant repercussions.

Their Content

Their content may look impressive but may not even be theirs, search on Google by placing a few random sentences in quotes and see what turns up.
If the content is original see how well it reads as some SEO service providers like to out-source and this is often detectable from the copy.

Strategic Plan

Without a long-term SEO plan to encourage natural linking a site is forced to use short-term tactics, I'd expect a decent SEO service to first look at the site itself to determine what additions are necessary to make it naturally link-worthy.

While neither fool-proof nor exhaustive these pointers should help you sort out at least some of the wheat from all the chaff out there.