There are many rumors surrounding the venerable IP address as it relates to search engine optimization and web hosting.  So let's talk about it, dispel some myths, and conclude with some information regarding hosting services that provide for hosting on various ranges of IP addresses.

It is useful to think of an IP exactly as you do a phone number.  Let's look at a phone number for a random "Mr. Cutts":

For this example's sake, let's format the phone number differently -- as if it were an IP:

650.949.4094.  The first number, "650," is the area code, the second, "949," is the exchange, and the third, "4094," is the number within that exchange.  The numbers go from most significant to least significant.  "650" probably indicates "somewhere in california."  "949" means "some county in california," etc.  So we can assert that the person answering the phone at 650.949.1234 is in the same neighborhood as 650.949.4567.

Of course this is not a sure thing since VOIP came along, but it's not a sure thing for an IP, either.  It's true -- in general.  Let's look at an IP address -- the IP address of this blog:

66.39.117.78

The first number, "66," is the "class A," the second, the "class B," and so forth. 

So why does this all matter?  Google (and likely other search engines) may use this information to discover relationships between web sites.  Matt Cutts alludes to this here.  IP addresses that are within the same "class C" range are very likely to be nearby, perhaps even on the same server -- just as with our phone number example.  A search engine can use this information to detect spammers, and devalue links that are among web sites in that particular range.

First, a word about what this doesn't mean:
This doesn't mean you need a dedicated IP address.  Matt Cutts demystified this here; in fact, having a dedicated IP is completely irrelevant.  The odds of many like-themed cross-linked web sites being on your IP address is low.  If you're curious about who else is on your IP, use this tool.

But perhaps we host quite a few sites with similar themed content and do not wish to worry about this.

This actually proves to be quite frustrating, since it's nice and convenient to host web sites with one vendor.  And who wants to risk disaster and host our sp ... web sites in one location?

Fortunately, there are a few vendors that noticed this niche and offer hosting in multiple class Cs.  Many of them also offer custom nameservers and netblock information.  Here's the list:

http://www.dataracks.net/:
Shared hosting, dedicated hosting, and colocation, different ownership information on each netblock (very paranoid).

http://www.gotwebhost.com/:
Shared hosting, dedicated hosting, VPS 

http://www.seowebhosting.net/:
Shared hosting, dedicated hosting, and colocation

http://www.webhostforseo.com/:
Shared hosting

http://www.datawebpro-seo-hosting.com/:
Shared hosting

Of course I wouldn't touch some of these with a stick, but use your judgment; a few of them look like professionals.  Leave a comment if you have any other hosting recommendations!