I was thinking of this when Thumbplay, a site for mobile entertainment downloads, sent over news of its SMS vertical search service. How does this fit in with all the ways consumers can search mobile devices? And where can advertisers reach these consumers?

There are five predominant ways consumers can search through mobile devices: on-deck, off-deck, applications, voice, and SMS. Most present media buying opportunities, while search engine optimization only factors into off-deck search. Beyond the five search vehicles, there are other emerging models we'll discuss briefly at the end.

On-Deck: Currently, most mobile search and Internet usage occurs on-deck - on the carriers' branded portals. The deck is exactly like AOL in the 1990s, where AOL focused on bringing brand-name content to the user in its walled garden. The Web pioneers weren't using AOL, and AOL's users gradually used its browser WebCrawler to start exploring the Web; before long the garden's walls disintegrated. The same evolution will happen from on-deck to off-deck. The default search engines on the carriers' decks are powered by mobile advertising companies like Medio Systems and JumpTap, which also run search and display for other publishers.

Off-Deck: When you go to Google or Yahoo on your mobile phone, that's searching off-deck. The main search engines have the biggest advantage here over the long-term, even if new and emerging rivals manage to effectively compete in specialized and vertical search fields. Today's leading engines have an extra edge with marketers, as it's getting easier to run integrated campaigns across the engines and their mobile versions.

Applications: Google and Yahoo both offer downloadable applications for mobile search, local search / maps, and other functions. That makes the engines easily accessible from smartphone handsets, and the applications offer additional functionality that wouldn't be readily available on scaled-down mobile sites.

Voice: Voice search isn't necessarily mobile search, as one can call from a landline, but it's most useful for consumers who aren't near a computer. Before I had mobile Web access, I used to love calling voice service Tellme (owned by Microsoft; dial 800-555-TELL) for sports scores and weather, and I still regularly call Jingle Networks' 800-FREE-411 or Google's 800-GOOG-411 for local business listings, especially when I need specific information such as a business's cross-streets or hours. While GOOG-411 is ad-free and Tellme focuses on its enterprise hosted voice platform, FREE-411 callers will hear targeted ads when they call. Mobile Web search could later compete with voice search, but right now both have room to grow as they steal market share from the Yellow Pages and paid directory assistance.

SMS: The consumer writes a text message to a service like GOOGL (46645), YAHOO (92466), or WLIVE (95483 for Windows Live), enters a request such as "pizza 60611," and the service responds moments later with relevant listings (in this case, hopefully the original Pizzeria Uno in Chicago). Although some of the commands take a bit of adjustment to get the most out of the system, it's fairly easy, it works on any handset, and the results are stored as text messages for future reference.

This brings us back to Thumbplay's news that it just launched an SMS search service for entertainment content like ringtones, games, videos, and wallpaper. Here, the user sends a text to 48000 with a command like "get elvis," Thumbplay returns a link via SMS which the user can click to see all the related content, which in this case includes mostly Elvis Costello ringtones. It's a hybrid SMS-mobile Web model rather than true SMS search, as one needs mobile Web access to fulfill it. Thumbplay presents an easier way to access deep content within a mobile site, and it's easy to complete a purchase in a few clicks.

Hybrid models are just one source of innovation. There are technologies like Veveo's vTap, which makes it easier to search for content like videos from any device, and ican be especially useful for mobile. Then there are directories like Pricegrabber's mobile site atpgw.com (you can try it from a standard Web browser), which lets you drill down by category until you find the product you want, and then there's a link to the phone number to call in an order. Aggregate Knowledge's discovery engine is a further leap from search, and it now extends to mobile via a partnership just announced with CBS Mobile.

It all seems rather dizzying compared to Web search, where the overall process for how consumers search has been standardized. When consumers are at their PCs, they don't email searches or speak into their computers; even searching through instant messaging, which has been around for years, has barely emerged as more than a novelty. Carriers, search engines, technologists, ad networks and others appreciate that mobile search is different, even as everyone's trying to figure out just how different it is. That means standardization for mobile is a long way off.