after hours of investigation, I found out that google search (AJAX) APIs DO NOT support the ability to spell check a string. Anyyay, there's a way to do it, via the google tool bar. All needs to be done is doing an XML HTTP Post right away to http://www.google.com/tbproxy/spell?lang=en&hl=en (being en = english / sp=spanish).

this is the XML you need to POST:
<?xml version="1.0" encoding="utf-8" ?> 
<spellrequest textalreadyclipped="0" ignoredups="0" ignoredigits="1" ignoreallcaps="1">
    <text>Ths is a tst</text>
and this is what google returns back:
<?xml version="1.0" encoding="UTF-8"?>
<spellresult error="0" clipped="0" charschecked="12">
    <c o="0" l="3" s="1">This Th's Thus Th HS</c>
    <c o="9" l="3" s="1">test tat ST St st</c>
o: the offset from the start of the text of the word
l: length of misspelled word
s: Confidence of the suggestion
text tab (chr(9)) delimited list of suggestions

Now, the good thing is that I found out how to make it to work via ASP/VBScript. All samples provided on the web have been just for C/Jscript/PHP . This is the VB code. Hope it helps
	Dim xmlDoc
	Dim xmlHTTP 
	Set xmlDoc = CreateObject("Microsoft.XMLDOM")
	tsearch = "ths is a tst"

	xmlDoc.async = False
	taux= "<spellrequest textalreadyclipped=""0"" ignoredups=""1"" ignoredigits=""1"" ignoreallcaps=""0"">" & _
	                "<text>" & tsearch & "</text>" & _
	xmlDoc.loadXML (taux) 'loads the XML from string

 	'depending on the server you are hosting this code, it could be "Microsoft.XMLHttp" or "MSXML2.ServerXMLHTTP"
	'Set xmlHTTP = CreateObject("Microsoft.XMLHttp")
	Set xmlHTTP = CreateObject("MSXML2.ServerXMLHTTP")	

	xmlHTTP.Open "POST","http://www.google.com/tbproxy/spell?lang=en&hl=en", False 	
	xmlHTTP.send xmlDoc.xml 'do the HTTP post out to google

	trevisedstring = xmlHTTP.responseXML.xml
	response.write trevisedstring

	'first word will be always be the one you need, ignore the rest
 	'now all you need to do is parsing the XML response as normally
	'you will probably start with something like:

	Set objXMLDoc = Server.CreateObject("Msxml2.FreeThreadedDOMDocument")
	objXMLDoc.async = false
	objXMLDoc.validateOnParse = False
	objxmldoc.loadXML (xmlHTTP.responseXML.xml)

	'do the parsing here for each <c> ... </c> node

	set objxmldoc=nothing
	set xmlHTTP=nothing
	set xmlDoc=nothing