are TXT (SPF) records global for sub-domains?

I have a SPF record for my top-level domain, structuredweb.com. see "dig" outputs below. as part of our operations we send emails from swmail.structuredweb.com (64.14.55.140) in which the Return-Path is an address under "inbound.structuredweb.com". i have no SPF record for this sub-domain.  when i run dig for this sub-domain i get no answer.
however, when i look at the headers of a typical email, i see that google approves this sender, as if it's honoring the SPF record for the top-level domain.

Is there any standard here? can I assume SPF records affect sub-domains unless there is a specific record on the sub-domain? the last example shows "campaigns.structuredweb.com" for which i do have a specific SPF record.

1) ========== email header ===========
Received-SPF: pass (google.com: domain of
 XCAM_xxx@inbound.structuredweb.com designates 64.14.55.140 as
 permitted sender) client-ip=64.14.55.140

2) ===== dig for txt record of structuredweb.com ============
$ dig @dns025.b.register.com structuredweb.com txt
; <<>> DiG 9.3.3rc2 <<>> @dns025.b.register.com structuredweb.com txt
; (1 server found)
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 3276
;; flags: qr aa rd; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;structuredweb.com.             IN      TXT

;; ANSWER SECTION:
structuredweb.com.      14400   IN      TXT     "v=spf1 a:swmail01.structuredweb.com a:swmail.structuredweb.com include:aspmx.googlemail.com ?all"

;; Query time: 31 msec
;; SERVER: 216.21.232.25#53(216.21.232.25)
;; WHEN: Fri Mar 26 11:20:24 2010
;; MSG SIZE  rcvd: 144

3) ===== dig of inbound.structuredweb.com =============
$ dig @dns025.b.register.com inbound.structuredweb.com txt
; <<>> DiG 9.3.3rc2 <<>> @dns025.b.register.com inbound.structuredweb.com txt
; (1 server found)
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 53828
;; flags: qr aa rd; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0

;; QUESTION SECTION:
;inbound.structuredweb.com.     IN      TXT

;; AUTHORITY SECTION:
structuredweb.com.      14400   IN      SOA     dns174.a.register.com. root.register.com. 2009070226 28800 7200 604800 14400

4) ===== dig of campaigns.structuredweb.com =============
$ dig @dns025.b.register.com campaigns.structuredweb.com txt
; <<>> DiG 9.3.3rc2 <<>> @dns025.b.register.com campaigns.structuredweb.com txt
; (1 server found)
;; global options:  printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 49336
;; flags: qr aa rd; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;campaigns.structuredweb.com.   IN      TXT

;; ANSWER SECTION:
campaigns.structuredweb.com. 14400 IN   TXT     "v=spf1 ip4:64.14.55.140/32 ip4:64.14.55.141/32 ?all"

;; Query time: 32 msec
;; SERVER: 216.21.232.25#53(216.21.232.25)
;; WHEN: Fri Mar 26 11:29:41 2010
;; MSG SIZE  rcvd: 109