Student pilot leaves TRACON frequency to coordinate with Tower; only to come in close proximity to other traffic.

Narrative

I am a student pilot; and was completing my first solo cross-country; flying VFR from Palo Alto (PAO) to Castle-Merced (MER) and back to PAO. When approximately 10 miles north of Mount Hamilton and descending from 6;500 to 3;500; NORCAL instructed me to overfly San Jose International airport. When I safely passed a departing air carrier; I was instructed to resume normal navigation to PAO. I descended to approximately 2;500 FT while approaching NUQ airspace. Not realizing that NORCAL could instruct me to transition NUQ airspace; I mistakenly went off-frequency (for approximately 45 seconds) and contacted NUQ Tower for permission to transition their airspace. I then immediately went back to NORCAL frequency and was informed that there was an aircraft directly below me; within 100 FT. I realized immediately after changing frequency that I should not have done so. I immediately climbed to 2;500 FT to avoid the other aircraft. NORCAL then instructed me to contact PAO tower; which I did. I was cleared for straight-in landing on Runway 31. The other aircraft; which was slower; was instructed to follow me for a straight-in landing on Runway 31.